1. Cell Division--2.Structure of Chromosomes--3.Principles of Genetics

8/2/2019 1. Cell Division--2.Structure of Chromosomes--3.Principles of Genetics

1/148

1. Cell Division


2/148

* Introduction* The need for new cells* The Cell Cycle

* Types of Cell Division1. Mitosis

2. Meiosis*Summary


3/148

Introduction

1. Definition of a Cell:2.Growth and Development:3. Organization:

4. Function of a cell:


4/148

The Need for New Cells

1).Growth

2).Replacement3).Repair

4).Reproduction


5/148

The CELL CYCLE


6/148

The Cell Cycle

To divide, each new cell has to undergo aphase of GROWTH and DEVELOPMENT.

It is after this phase that the cell attainsenough maturity and can complete allmetabolic processes that are necessarybefore entering the phase of cell division.

This preparatory phase of cells is termed asInter-phasefollowed by cell divisionMITOSIS


7/148

INTERPHASE

It is the long phase in a body / somaticcell.

Before dividing, each cell has to attain thesame size as its mother cell.

It has 3 sub-stages:

a). G-1 Phaseb). S Phase (Synthesis Phase)

c). G-2 Phase


8/148

G-1 PHASE

Its an initial growth phase or first growthphase of the cell.

The amount of cytoplasm increases and cyto-

plasmic organelles like mitochondria etc.replicates.

S-PHASE (Synthesis PHASE)-Chromosomes are duplicated by the synthesis

of more DNA


9/148

G-2 PHASE This growth phase is shorter. Proteinrequired for cell division are synthesized.

Cell increases in size. Cells prepares itself to enter the next phase of

division Mitosis The nucleus of cell enlarges and a definite number

of fine, coiled thread like structures calledchromosomes become visible.

In fact at the beginning of mitosis the nuclearmaterial has already doubled. This stage is calledINTERPHASE, also called the Resting Stage when infact the cell is synthesizing the chromatinsubstance.


10/148

CELL CYCLE


11/148

What happens in INTERPHASE?

Cellsizeincreases.

Nucleusof dividing cellenlargesto its maximumsize.

Chromosomesreplicatei.e. a copy of eachchromosome is created.

Nucleolusisvery clear.

Theenergyfor cell division is stored as proteins.

Certainsubstancesnecessary forinitiationofcell division are formed.


12/148

Cell Division When the cell has reached certain level of growth

it enters the division phase Where through a sequence of events the mature

parent cell divides into two daughter cells.

These phases are of various durations in differentcells, but the basic outline of sequences is thesame.

Cell division consists of two major stages:

1). Karyo-kinesis: Nuclear division where nucleusdivides into two.

2). Cyto-kinesis: Entire cells splits separating eachnuclei.


13/148

Types of Cell Division

Cells in animals and plants divide in oneof the two ways described below:

1). Mitosis: that takes place in body cellsresulting in growth and development.

2). Meiosis: that takes place in gamete

producing cells resulting in gametes

that fuse forming zygote.


14/148

MITOSIS Mitos = Thread and Osis = State Cell division process was first described by

SCHLEIDEN. This process takes place in Body or Somatic

Cells of organisms. The basic principle is The Chromosomenumber of cells is not altered (notchanged) in this process. Its justduplication.

Its a complex process where mother / parentsomatic cell divides to form two identicaldaughter cells.


15/148

Phases of Mitosis

1). Prophase2). Metaphase

3). Anaphase

4). Telophase


16/148

1). Prophase- Pro means First

- Nucleus is visible withlarge, shortened much likespring & thickenedChromosomes.- Nuclear Membrane &Nucleolusdisappear.- Centrioles two minutebodies outside the nucleusseparates & forms spindlefibres or astral rays &attaches to centromere of

chromosomes.- Most plant cells do nothave centrioles but stillhave a spindle network


17/148

AsPROPHASEendsfollowingfeaturesare noticed

Chromosomes: thick, clearly visible, eachchromosomes split into 2 chromatids & remainin contact throughout at the centromere.

Nucleolus & Nuclear Membrane startsdisappearing.

Continuous spindle fibresjoin both centrioles.

Chromosomesattach to spindle bycentromeres and start moving towardsequatorial position.


18/148

KINETOCHORE is the site

of attachment of spindlefibres.

CENTROMERE is the

center of attachment ofthe chromatids


19/148

2). Metaphase

- Metameans After

- Short phase- Chromosomesbecomes most distinct.

- Duplication of

Chromosomes occursbefore prophase, butbecomes more evidentduring metaphase.

- Chromosomesarrange themselves atthe equatorial plane.

A B k


20/148

3). Anaphase- Anameans Back

- Very active phase ofMitosis. Main features are

a). Homologous daughterchromosomes (2chromatids) of metaphasenow separate and begin tomove in the opposite

direction towards the polewith the contraction ofspindle fibres.

b). Chromosomes bend

around the centromereshowing as V or J or Lshaped structure based onthe length of its arms.


21/148

4). Telophase

- Telo means End

- As soon as thechromatids cometogether at the endsof the spindle

network, thetelophase begins.

The various changes observed during


22/148

The various changes observed during

TELOPHASE are:1). Chromosomes reach opposite poles. Spindle

network disintegrates but centrioles remain.2). Chromosomes uncoil, lengthen & thus becomes

thinner turning into a network of less distinctchromatin threads.

3). Nuclear membrane & Nucleolus startsforming around 2 sets of chromosomes inboth the newly formed daughter cells.

4). With the formation of nuclear membrane, 2 nucleiare formed & the process of nuclear division orKARYO-KINESIS is over. This is followed byCYTO-KINESIS.


23/148

Cyto-Kinesis

- At this point the cytoplasm

between the two daughternucleiconstricts i.e. afurrow starts forming in theplasma membrane at themiddle.

- This deepens towards theinterior of the celluntil thecytoplasm is split into 2similar daughter cells.

- In plant cells constriction isnot observed duringcytokinesis.


24/148

Difference in Plant and AnimalMitosis

PlantMitosis

1. Centrioles are absent inPlant Mitosis.

2. No contractile ring is found.

3. No cell constriction isformed.

4. In Telophase there is aformation of Cell Plate for

the process of cytokinesis.5. Nuclear and Cell division

are found in special regionscalled Meristems.

AnimalMitosis

1. Centrioles are present inAnimal Mitosis.

2. Formation of contractile ring

between 2 nuclei of thedividing cell

3. Cell constriction is formed inthe late Telophase for the

process of Cytokinesis.4. No cell plate is formed.

5. Cells divide everywhere, allthe time


25/148

Importance of Mitosis

- Helpful in growth of tissues, organsand thus organism.

- Used for repair of damaged tissues andorgans.

- It is the method by which lower animalsreproduce.

- Hereditary characters are maintainedby the replication of chromosomes.


26/148

Characteristics of Mitosis

- It takes place in Somatic / Body cells.

- It helps in tissue growth.

- 2 equal similar daughter cells areformed.

- Chromosome number is preserved.

- Daughter cells are identical to parentcell.

M i i


27/148

Meiosis- Meiosis means To reduce.

- It is a specialkind of cell division that takesplace in reproductive / germ cellsonly.

- Cells in the reproductive tissuesmultiply innumber by mitotic divisions.

- The final division that producesmaturegametes however, are not mitotic.

- In fact these division are of the reducing

type.- The number of chromosomes in the cell is

halved in gametes.


28/148

- When these haploid gametes fuse to forma zygote the diploid chromosome numberof organism is restored.

- In man, meiosisoccurs in testes producing

sperm and in female it occurs in ovariesproducing ova.

- In plant kingdommeiosis occurs in anthers

and ovaries of angiosperms (floweringplants) producing pollen grains and ovulesrespectively.

In Meiosis the cell divides in two


29/148

In Meiosis the cell divides in two

sequences as below1). Heterotypicor

1st Meiotic Division- Here the diploid

chromosome number ofparent cell is reduced to

half (haploid) in daughtercells.

2). Homotypicor2nd Meiotic Division

- This is normal mitoticdivision with no change inthe number ofchromosome.


30/148

1).Heterotypic Division (Meiosis-I)

It is also known as First Meiotic Division

Here the chromosome number is halved.

It consists of following phases1. Prophase I

2. Metaphase I

3. Anaphase I4. Telophase I


31/148

All these phases be summed up as under:-

a). Pairing of Chromosomes:Homologous chromosomespairs seem to attract each other. They are similar

chromosomes-one each from either parent. Each pair ofhomologous chromosomes is also known as tetrad becauseit has four chromatids.

b). Crossing Over:While the homologous chromosomes are inthe tetrad formation the chromatids seem to cross each other

at one or at more points. The chromatids actually break andrejoin at these points of intersection known as Chiasmata.Alongs with parts of chromatids exchanged, genetic materialgets recombined into new combinations of genes.

c).Nuclear envelope, nucleolietc disappearand the

chromosomes move apart to the opposite poles and 2 newcells are formed. Thus at the end of first meiotic division, 2haploid daughter cells are form from a diploid parent cell.


32/148


33/148

Significance of Crossing Over

- During Prophase-Icrossing over takes place.- The maternal and paternalhomologouschromosomes of an organism pair up andexchange certain portions of their chromatids.

- This crossing over is the basis of geneticvariations in organisms.

- This is why all offspring of 2organisms are notalike.

- Crossing over makes the genetic informationin each haploid cell unique.

- Each chromosome pair can crossover at many pointsmore than once.

2) H t i Di i i (M i i II)


34/148

2). Homotypic Division (Meiosis-II)- This division is identical to Mitosis.- Here the chromosome number (haploid) is maintained

and at the end, four haploid daughter cells are produced.- The phases of second meiotic division are divisible into

Prophase-II, Metaphase-II, Anaphase-II and Telophase-II.

- Cytokinesis in meiosis varies greatly.- Sometimes two daughter cells produced at end ofMeiosis-I undergo only Karyo-kinesis and go throughMeiosis-II before dividing into four haploid daughtercells.

- At other times Cytokinesis occur at end of Meiosis-I andthus two haploid daughter cellsproduced under goMeiosis-II.


35/148

Characteristics of Meiosis

Occur in germ cells only. Results in formation of gametes.

Results in formation of four haploid

daughter cells.

Chromosome number of parent cell is

halved. Daughter cellsdiffer from parent cell.


36/148

Significance of Meiosis

Due to Meiosis, chromosome number isreduced to half in gametes andchromosome number of species ismaintained.

Due to crossing over in Prophase-I, geneticvariations occur.

Due to Meiosis, the zygote on fertilization

has one maternal and one paternalchromosome in each homologous pair.

Summary


37/148

Summary Cell division is of2 types in higher plants and animals

1). Mitotic 2). Meiotic

Amitosis is the simplest cell division found in groupProtista. E.g. Amoeba, Paramoecium etc..

first: nucleus divides (karyo-kinesis)

second: cytoplasm divides (cyto-kinesis) Mitosis

- it is helpful in growth and repair oftissues

- it results in 2 daughter cells identical to parent cells in

all aspects- it takes place in somatic / body cells

- it takes place in one phase.

S


38/148

Summary Meiosis

- it takes place in 2 phases.- it occurs in germ or reproductive cells.- it results in the formation of 4 daughtercells different from parent cells.

- it results in formation of gametes forreproduction.- it is called reduction division becausechromosome number of parent cell is

halved in daughter cells.- Crossing over in Meiosis is responsible forgenetic variations in individuals.


39/148

CHAP. 2.Structure of Chromosomes


40/148

1. Introduction

2. Discovery of Chromosomes3. Basic structure of

Chromosomes4. Significance of Chromosomes5. The DNA Strand

6. Importance of DNA7. Summary

Introduction


41/148

Introduction Chromosomes are the chromatin

material inside the nucleus. Chromosomes are so called becausethey take up certain basic dye stains

very readily. Chromos = colour and soma =body

Chromosomes consist of Histoneproteins and DNA (deoxy-ribonucleicacid).


42/148

DNA carry information for

synthesis of various proteinsrequired for physiological

functions. These stretches of DNA are

referred to as GENES

Chromosomes are best observedat Metaphase stage.

l h


43/148

Colchicine

(0.3 ml of 1 % Colchicine)

It is used to stop cell cycle during Metaphase.

It is used

To check defects in lengths of arms

To ensure attachments of spindle fibres for

correct separation.

Colchicine is used in chromosome

Karyotype analysis.


44/148

Definition of Chromosome

Chromosomes are chromatin materialthread like structure present inside the

nucleus made of DNA and Histone

proteins and are important in transfer ofgenetic material (genes) from one

generation to the future generation.

Also termed as Hereditary material.

Discovery of Chromosomes


45/148

Discovery of Chromosomes E.Strasburger: Observed thread like structures

during cell division. Balbiani: Described rod like structures in nucleusbefore cell division.

W. Fleming: Described splitting of chromosomecalled stained material Chromatin

Waldeyer: Coined the term Chromosomes

Sutton & Boveri: Said chromosomes are physicalstructures and transmitters ofhereditary traits


46/148


47/148

TELOMERE: A TELOMERE is a region of

repetitive DNAat the end of a chromosome,

which protects the end of the chromosomefrom deterioration. Its name is derived from

the Greek words telos "end" and mers

"part". The telomere regions deter the

degradation of genes near the ends of

chromosomes by allowing for the shortening

of chromosome ends, which necessarily

occurs during chromosome replication.

CENTROMERE A C t i


48/148

CENTROMERE: A Centromere is a

region ofDNA typically found near the

middle of a chromosome where twoidentical sister chromatids come

closest in contact. It is involved in celldivision as the point of mitotic spindle

attachment. The sister chromatids are

attached all along their length, but

they are closest at the centromere.


49/148

HISTONES: Histones are stronglyalkaline proteins found in eukaryoticcell nuclei, which package and orderthe DNA into structural units called

nucleosomes. They are the chiefprotein components of chromatin,act as spools around which DNA

winds, and play a role in generegulation.

Basic Structure of Chromosomes


50/148

Basic Structure of Chromosomes1. Chromosomes are thread like bodies

presentin nuclei of animals and plants.2. They are covered with a SHEATH made of

proteins.

3. Inside this sheath is present a granularmatter referred as matrix.

4. Inside the matrix there are two threads

called Chromonemata which are thesubunits of chromatids & are presentduring Prophase.


51/148

5. At Metaphase, the chromosome consists


52/148

p ,of two symmetrical strands calledChromatids.

6. Each chromosome possess a distinctconstriction called Centromere (Primaryconstriction), divides into 2 parts and it

gets attached to the spindle network.7. The ends of chromosome are termed as

Telomeres and it protects from

deterioration orfrom being destroyed.8. Some chromosomes have anotherconstriction called Secondaryconstriction


53/148


54/148


55/148


56/148

Chemical composition of chromosome

Substance AmountDNA 40 % approx

Histone proteins 50 %

Non-Histone proteins 8.5 %

Metallic ions like Mg++,Ca++ etc.

In traces

DNA packaging in Chromosome


57/148

DNA packaging in Chromosome

Ch h k i d ili


58/148

Chromosome: The packaging and coiling

The chromatin fibre is actually a very long DNA

strand coiled again and again.

The coiling is strictly according to the plan.

The DNA strand makes loops around apolymer

of 8 histone protein molecules.

Each such histone & DNA unit is called a

nucleosome

This coiled strand is coiled repeatedly, much like

a telephone cord till it becomes the

chromosome familiar.

Chromosome NumberCommon Name Chromosome No.


59/148

Chromosome NumberCommon Name Chromosome No.

Animals:

1. Round worm 2

2. Mosquito 6

3. Fruit Fly 8

4.Human Beings 46

5.Butter Fly 446

6.Dog 78

7.Monkey 54

8.Mouse 40

Plants:

1. Bread Mould 2

2. Onion 16

3. Wheat 42

4. Sugar Cane 80

5. Grass 265

The number of

chromosomes isconstant for a particular

species.

Each species has its

own chromosome no..

Size of organisms hasno relation with its

chromosome numbere.g. Butterfly has 446

chromosomes than Man

(46)

Chromosome Size


60/148

Chromosome Size

Chromosome size is usually measured at

metaphase when they are clearly visible. Plant generally have larger chromosomes as

compared to animals.

Chromosomes of Monocotyledonous plants arelarger than Dicotyledonous plants.

Chromosomes have a size range of0.20 0.50m

Chromosome vary in size from species to speciesand remain relatively constant for a particularspecies.

Human chromosomes are up to 6m in length.

Chromosome Shape


61/148

Chromosome Shape Usually observed at mitotic

anaphase. The shape ofchromosome may be:

1). Telocentric: centromere at1 end makes chromosomerod shaped.

2). Acrocentric: centromerenear 1 end divides it into

very short and a long arm.3). Submetacentric:

centromere near centregives L or J shapedstructure.

4). Metacentric: centromere atcentre giving V shapedstructure.


62/148

Depending upon the position of


63/148

p g p p

centromere

MONOCENTRIC: If chromosome hasonly1centromerethen it is calledMONOCENTRIC.

DI-CENTRIC: while if there are 2centromeres, it is calledDI-CENTRIC

HOLOCENTRIC: If centromere isdiffused then the chromosome iscalled HOLOCENTRIC.

POLY CENTRIC if th


64/148

POLY-CENTRIC: if there are

more centromeres, thechromosomes is known as

POLY-CENTRIC. ACENTRIC: If centromere is

absent, the chromosome isknown as ACENTRIC

Karyotype: A species is characterized by a set of


65/148

Karyotype: A species is characterized by a set ofchromosomes whose features are constant in

individuals of the same species. This set of

characteristics is called a Karyotype. OR

The appearance of the chromosomal makeup of a

somatic cell in an individual / species (including the

number, appearance, arrangement, size andstructure of the chromosomes)

Idiogram: A Karyotype is depicted by a diagramusually. Here chromosomes of a haploid set arearranged in decreasing order of size. This is called anIdiogram.

Karyotype of a Human:


66/148

Karyotype of a Human:

Chromosomes in Man


67/148

The chromosome number in Man is 46 i.e. 23pairs.

Out of 23 pairs, 22 pairs are common in bothmales and females. These are calledAutosomes.

23rd pair of chromosomes is a small pair and it

determine the sexin the man. In male, 22 pairs are autosomes and 23rd pair

are sex chromosomes that are denoted by X &Y(22pairs of A + XY).

Similarly in females, 22 pairs are autosomesand 23rd pair are sex chromosomes but theyare identical i.e. there are 2 X chromosomes(22 pairs of A + XX).


68/148

- The X and Y chromosomes are

different in shape and size.

- Y chromosome is Jshaped and issmaller than X chromosome. But

they pair up during meiosis.

Significance of Chromosomes


69/148

g1. Chemicals of chromosome remains stable

even though the constituent chemicals of

cytoplasm of a cell are broken down.

2. During Cell division, no other material ofcell is shared as exactly as chromosomes.

3. They are the main source of chemical information that determines that a cellshould become like its parent cell.

4. They give the characteristic features of itsspecies during development.

5. The nuclei of gametes carry a set ofpaternal


70/148

and maternal haploid chromosomes whichunite to form diploid zygote and give rise to a

unique individual.

6. The number of chromosomes is also

important. Sometimes addition or deletionof chromosomes in zygote can cause serious

disorders in the individual (MUTATION).

7. Sex chromosomes are also of greatsignificance as they determine the sex of an

individual.

DNA Strand


71/148

Chromatin material is basically the DNA strand

wrapped around Histonesforming nucleosomes.This coiled strand is further coiled to form

chromatin filament.

DNA itself consists of two filaments or strandshelically coiledwith each other.

It is also called a macromolecule because it is a

very large single molecule.The detailed structure of DNA was worked out by

Watson & Crickin 1953.


72/148

Watson and Crick DNA


73/148

Watson and Crick DNA

model

- Each DNA strand has acomplex chemical structure.

- It is made up small building

units called nucleotides.-Each spiral of DNA has 10

nucleotides.

- Each nucleotide consists of 3

subunits (one molecule each

of pentose sugar, phosphate

and nitrogenous base).


74/148

-The Phosphate and Sugar molecule are


75/148

p garranged one above the other alternately whilethe base is oriented inwards, attached to ribose

sugar.-The base of one strand joins with the base of

the complementary strand with Hydrogen-bond

forming a spiral staircase where the basesforms the steps.-The nitrogenous base join only with its

complementary base thus Adenine (A) joins

with Thymine (T) through double bond &Cytosine (C) joins with Guanine (G) throughtriple bond i.e. A=T and CG


76/148

Main Features of DNA


77/148

There are 2 strands running anti-parallel to eachother & are spirally arranged around an axis so it is

called a double helical arrangement.Each DNA strand is made up of sugar, base &

phosphate.

Sugar are pentose type andbases are of 2 types:

1). Purines:Adenine (A) & Guanine (G).2). Pyrimidines:Cytosine (C) & Thymine (T).

Bases are connected with each other throughHydrogen Bondsi.e.A=T and GC.

Total number ofPurines = Pyrimidines


78/148

i.e. A + G = T + CBase ratio i.e. (A + T) / (G + C) is constant for a given

species. This ratio is called Dissymmetry Ratio.

Bases are inside and sugars are outside and 2 sugars

are connected with Phosphoric Acid.

DNA has a property of Duplication. Each replicates

and from each replicated DNA another new DNA is

formed. This is called Replication of DNA.

Before Replication of DNA, Hydrogen bondsbetween the bases are broken.


79/148

Importance of DNA

Very important because it is Hereditarymaterial.

It acts as director ofProtein Synthesis.

Some DNA of chromosome forms

nucleolus.

DNA in presence of enzymes formmRNA which acts as messenger.

DNA Replication


80/148

DNA Replication

During Cell Cycle chromatin material

duplicated during inter-phase in order toprepare formitosis.

In this process, DNA double helix open up at

one end.When the 2 strands become free at one end,

new nucleotides start forming

complementary pairs and thus new strandsstart forming around each unraveling oldstrand sequentially.

DNA Replication


81/148

DNA Replication

In the replication process the parent DNA molecule

unwinds and unzips. Then each of the old strands serves as the

template for the new strands.

Each daughter DNA molecule receives one parentalstrand and a newly synthesized strand.

This type of DNA replication is commonly called as

semi-conservative replication, because here eachdaughter DNA molecule receives one parental

strand.


82/148

DNA replication requires following three steps:


83/148

Unwinding: - The old strand that makes up the parent DNAmolecule is unwound and unzipped (weak hydrogen bonds

between the paired bases are broken). The hydrogen bondsbetween the molecules are broken with the help of Helicaseenzyme.

Complementary base pairing:- With the help of enzyme DNApolymerase new complementary nucleotides (that arealways present in the nucleus) are positioned adjacent toeach other opposite to the parent DNA template.

Joining:-This step also requires DNA polymerase enzyme for joining the complementary nucleotides. Each daughtermolecule contains an old and a new strand.

Replication of DNA strand has an origin point at which thereplication is initiated. It may also have a terminus point


84/148

p y pwhere the replication of DNA is terminated. A ' Y ' shapedstructure is formed at the point of replication which iscalled as "replication fork ".

Replication of DNA may be unidirectional or bidirectional.During DNA replication, one nucleotide is joined withanother. Each nucleotide already has a phosphate groupat the 5' carbon atom and it is joined to 3' carbon atom of

the sugar molecule. Thus the synthesis of DNA molecule takes place in the

5'->3' direction with the help of DNA polymerase enzyme.But this causes a problem at the replication fork whereonly one of the new strands run in the 5'->3' direction( the template for this strand runs in the 3'->5' direction).This strand is called as LEADING STRAND.

The template for the other strand runs in the 5'->3'direction, but DNA synthesis could only take place in


85/148

, y y p

5'->3' direction. Thus, this poses a problem and due tothis reason synthesis has to begin in the replication

fork.

Replication of the 5'->3' parental strand begins assoon as the DNA molecule unwinds and unzips

replication of this strand is discontinuous. Thereplication of this strand results in segment calledOKAZAKI FRAGMENTS.

Discontinuous replication takes more time thancontinuous replication therefore the new strand inthis case is called the LAGGING STRAND.


86/148

DNA R li ti


87/148

DNA Replication


88/148

Summary Chromosomes are thread like structures in nucleus


89/148

Chromosomes are thread like structures in nucleus.

They are studied generally at Metaphase stage of Mitosis.

Chromosomes are made up ofDNA & proteins. Chromosome number (2n) of species is constant.

Chromosomes may be rod shaped / V / J / L shaped.

DNA is the hereditary material.

It act as director ofProtein synthesis. There are 2 nucleic acids DNA & RNA.

Man has 46chromosomes i.e. 23 pairs.

Chromosomes Common to males and females are referredto as Autosomes (22 pairs in human).

Chromosome determining the sex of an individual are calledsex chromosomes (XY in males & XX in females).


90/148

1). Introduction2) Gregor Johann Mendel


91/148

2). Gregor Johann Mendel

3). Mendels Experiments

4). Terminology5). Monohybrid Cross

6). Di-hybrid Cross

7). Mendels Laws8). Exceptions to Mendels Law

9). Sex Chromosomes in Human

10). Determination of Sex11). Sex Linkage

12). Sex Linked inheritance of diseases

Introduction


92/148

It is commonly observed that children resemble their

parents.

An individual inherits the characteristic features of the

species from its parents.

This resemblance is because the offspring inherits

certain Factors from the parents.

All the characters that offspring can inherit from the

parents are called Hereditary Characters and the

process of their transfer is called Heredity. Thus height, types of leaves, flower color, shape, seed

structure etc..are hereditary characters in plants.

- In the same manner curly hair, eye color, colorblindness etc in man are the hereditary characters


93/148

blindness etc.in man are the hereditary characters.

- These factors are now know as genes that are located

at specific points on chromosomes.- These genes control a particular Character or Trait.

- Heredity: means transmission of genetically based

characteristics from parents to offspring.- Each gamete has a complete haploid set of these

genes.

- Thus, the zygote inherits one half of its geneticmaterial from either parent. This is the one reason

why children resemble both the parents.

Although offspring resemble their parents,they are rarely identical to them and they


94/148

they are rarely identical to them and theyshow some dissimilarities from their parents.

Such dissimilarities in a species are called.

These differences are caused by genetic /

inherited variations and environmental / noninherited differences.

Only inherited variations can be passed to

offspring and have an influence in changingthe genetic constitution of the species.

Gregor Mendel


95/148

Know as Father of GENETICS

Born in 1822 in Czechoslovakia.

He carried out expts on garden pea for 8 yrs.

On the basis of his results, he derived a fewfundamental principles.

He presented his results in a paper Expts in PlantHybridization before Brunn Natural History Society in1865.

Mendel repeated his work with some other plantsalso but failed to repeat his results.

Hugo de Vries, Carl Correns & Erich von Tschermakworking independently rediscovered his work andbrought Mendels expts to the limelight.

Reasons for Mendels Success


96/148

He chose the garden pea for his expts.

Pea plants were easy to cultivate, had a short

lifespan and showed self-pollination as well

as easily distinguishable characters. This

helped him.

His success was also due to his meticulous

planning and laborious observations andrecord that enable him to have enough data

to be analyzed statistically.

Mendels Experiments


97/148

p

& Techniques

Hybridization (crossbreed) involves

crossing of two

individuals with

different desired

characters to produce

an offspring that has

desired characters ofboth parents.

New terms


98/148

New terms

Homozygous: refers to having identicalalleles for a single trait.

Eg. The gene for seed shape in pea plants

exists in two forms, one form or allele forround seed shape (R) and the other for

wrinkled seed shape (r). A homozygous plant

would contain the following alleles for seedshape: (RR) or (rr).

Heterozygous: refers to having two different

alleles for a single trait


99/148

alleles for a single trait.

E.g.The gene for seed shape in pea plants existsin two forms, one form or allele for round seedshape (R) and the other for wrinkled seed shape(r). A heterozygous plant would contain thefollowing alleles for seed shape: (Rr).

Organisms have two alleles for each trait. Whenthe alleles of a pair are heterozygous, one isdominant and the other is recessive. Using the

previous example, round seed shape (R) isdominant and wrinkled seed shape (r) is recessive.Round: (RR) or (Rr), Wrinkled: (rr).

Picture showing Homozygous andHeterozygous


100/148

Heterozygous

Mendels experiment


101/148

He cultivated the pea plants with different

characters and studied till he obtained TRUEBREEDING / HOMOZYGOUS plants (TT). E.g.

Homozygosity for tall plants was tested by

growing seeds of tall plants [ gametes of purebreed tall plants are TT and for dwarf plants

are tt ].

Before experimenting he considered some

characters for his experiments.

Characters Mendel considered are


102/148

1. Stem length: Tall / Dwarf

2. Flower position: Axial /

Terminal

3. Flower color: Red / White

4. Pod color: Green / Yellow

5. Pod shape: Inflated /Constricted

6. Seed shape: Round /

Wrinkled

7. Cotyledon color: Yellow /

White

The next step was HYBRIDIZATION


103/148

He chose 2 parents with alternate forms of a

character. Eg. Red / White flower color, Tall /Dwarf stem length etc..

Then he removed the anthers of the plant, he

designated female, dusted its stigma withpollen of desired male and bagged it.

Such a cross was called

because it involved cross between alternateforms of one character only. The offspring was

F1 generation.


104/148


105/148

Further

# Th M d l d i l h


106/148

# Then, Mendel made a reciprocal cross where

the parents in the earlier cross were reserved

i.e. male as female and vice versa.

# In all these, the offspring demonstrated Red

flowers showing dominance of Red color.# The F2 generation was obtained by bagging

complete flowers of F1 generation enabling

them to self pollinate and produce seeds.

TERMINOLOGY


107/148

Term Explanation Example

GENE The basic unit ofinheritance for a given

character

Height of theplant

ALLELE Alternate forms of thesame gene which

determine contrasting

characters

Tall (T) or

Dwarf (t)

Homozygous Diploid condition whereboth the alleles are

identical

TT or tt

Heterozygous Diploid condition whereboth the alleles are

Tt


108/148

both the alleles are

different

Phenotype The physical or external andobservable expression of a

character

Tall,Dwarf

GenotypeThe genetic expression of a

character in terms of alleles

written in symbols

TT, tt, Tt

Dominant An allele which expresses

itself externally whenpresent in homozygous or

heterozygous conditions

TT or Tt

(both representtallness: T is

dominant)

Recessive An allele which expressesitself externally when

t or dwarf


109/148

itself externally when

present in homozygous

condition but remainssuppressed in heterozygous

condition.

Monohybrid When only one pair ofalleles is used during

hybridization.

Tall x Dwarf

(TT) x (tt)

F1

Generation

The generation produced by

crossing two parental stocksis called first filial

generation.

(P) TT x tt

(F1) Tt

F2

G ti

The generation

produced by crossing(P) TT x tt


110/148

Generation produced by crossingtwo individuals of F1

generation is calledsecond filial

generation.

(F1) Tt x Tt

(F2) TT Tt Tt tt

Test Cross A cross between therecessive parent and

an individual of F1

generation.

(P) tt x Tt (F1)

Definition


111/148

Variation: Small differences betweenindividuals due to inheritance

Mutation: Sudden changes in one or more

genes in the progeny, which normally maynot have existed in the parents, grand

parents or even great grand parents.

E.g.Albinism (total loss of skin pigment)

Monohybrid Cross


112/148

Monohybrid cross is one where the parents used

for hybridization differ in only one pair ofcontrasting characters or allele.

Eg: Take Homozygous / pure TALL (TT) & pure

DWARF (tt) plant. The initial plant materials are called parents or P.

These are cross pollinated using emasculation and

bagging. The seeds obtained are sown and plants so

obtained form the F1 or first filial generation.


113/148

All plants of F1 generation are tall, the dwarf characterdoes not appear.


114/148

The character appearing in F1 generation is

DOMINANT and one that is suppressed is RECESSIVE. When plants of F1 generation i.e. Tall hybrids are

allowed to self pollinate the F2 generation is

predominantly tall with some dwarf plants.

Always the ratio of 3 tall to 1 dwarf plant

approximately is observed without any exception.

On allowing F2 plants to self pollinate dwarf plants

produce only dwarfs, 1/3rd of tall plants produce only

tall plants and remaining 2/3rd of hybrid tall plants

again produce tall and dwarfs in 3:1 ratio.

Di-hybrid Cross


115/148

In a dihybrid cross, 2 pairs of alleles or contrasting

characters are considered in parents. In his expt., Mendel used cotyledon color and seed

shape of pea as 2 pairs of allele

He followed the same method as in monohybrid cross

of the two parents one was a true breeding dominant

(yellow colored cotyledons and round seeds, YYRR)

and the other was a true breeding recessive (green

colored cotyledons and wrinkle seeds, yyrr).


116/148


117/148

-

1). YYRR: Yellow Cotyledons Round Seeds

2). YYrr: Yellow Cotyledons Wrinkled Seeds

3). yyRR: Green Cotyledons Round Seeds

4). rryy: Green Cotyledons Wrinkled Seeds


118/148

This gave an approximate phenotypic ratio of

9:3:3:1, which is know as di-hybrid ratio.

These results show that though dominant and

recessive alleles were present in same plant in

generation P, in F2 they separated independentlyform new combination of Yyrr and yyRR.

Thus, each pair segregate independent of each

other resulting in new combinations.

MENDELs LAW


119/148

MENDEL s LAW

1). LAW OF DOMINANCE: In a givencross between 2 organisms with

pure contrasting alleles or

characters only one allele isexpressed in F1 generation; that

character that appears is called

dominant and the other recessive.

Law of Dominance


120/148

) h


121/148

2). LAW OF SEGREGATION: When a

pair of contrasting alleles cometogether due to hybridization in a

hybrid individual, the 2 characters

remain together without mixing or

losing their purity. They segregate

(separate) in the gametes of thehybrid.


122/148

3).

When the parentsdiffer in two or more alleles then

the inheritance of one pair of alleles

does not depend on the other pair.

EXCEPTION OF MENDELs LAW


123/148

1).Unlike Mendels F1 generations, in afew cases F1 generation has an

intermediated phenotype betweendominant and recessive alleles e.g. inor

heterozygous F1 Rr flowers are pink as

compared to homozygous P (RR) redand P (rr) white.

Incomplete Dominance


124/148

Incomplete Dominance


125/148

2). Linkage:

G h id


126/148

Genes on same chromosomes are said to

be linked. The closer they are stronger thelinkage. Linked genes are inherited

together.

Mendel was lucky that none of his 7 alleles

showed linkage or his law of independent

assortment would not have been

formulated.

3). Multiple Allelism:

Each character may have more than 2 alleles


127/148

ye.g. rabbits fur color has alleles. This cant

be explained by Mendels laws which statethat each character has 2 alleles only.

4). Gene Interactions: (Epistasis)

Sometimes a character (e.g. complexion inhumans) is controlled by more than 1 geneor factor. This again is an exception to the

Mendelian statement that each character iscontrolled by one gene / allele.

Gene Interaction


128/148

1) D i d i h b


129/148

1). Dominant and recessive characters can be

found.2). A hybrid with desired characters can be

produced easily on the basis of Mendels laws.

3). Crops can be improved.

4). Pure recessive characters can be used where

needed.

5). Genotypes and phenotypes of next generation

can be predicted even before cross is made.

Sex Chromosomes in Humans The chromosome number in Man is 46 i.e. 23

i


130/148

pairs.

Out of 23 pairs, 22 pairs are common in bothmales and females. These are calledAutosomes.

23rd pair of chromosomes is a small pair and it

determine the sexin the man. In male, 22 pairs are Autosomes and 23rd pairare sex chromosomes that are denoted by X &Y (22pairs of A + XY).

Similarly in females, 22 pairs are Autosomesand 23rd pair are sex chromosomes but theyare identical i.e. there are 2 X chromosomes(22 pairs of A + XX).

Sex Chromosomes in Humans

The X and Y chromosomes are different in


131/148

The X and Y chromosomes are different in

shape and size. Y chromosome is J shaped and is smaller

than X chromosome but they pair up during

meiosis. From the given information it is clear that

maleness is due to presence of one X and

one Y chromosome and femaleness ispresence of two X chromosomes.

Determination of Sex

During meiosis the sex chromosomes are


132/148

During meiosis, the sex chromosomes are

also reduced to half like Autosomes forminga haploid gamete.

In female, ova will have one X chromosome +

22 Autosomes making the haploidchromosome number 23.

But in male, sperms half will have one X

chromosome + 22 Autosomes and the otherhalf will have one Y chromosome + 22Autosomes.


133/148

In humans and many other species males produce

two types of gametes i.e. X and Y, hence they are

Heterogametic.

Females produce only one type of gametes i.e. X,hence they are Homogametic.

Sex determination in Human


134/148

Sex Determination in Human


135/148

Sex Linkage


136/148

Sex chromosomes (X & Y) carry sex determining

genes i.e. male & female. But they also carry certain other genes that control

some important traits.

Such characters / traits that are controlled bygenes occurring on sex chromosomes are calledsex-linked characters.

The genes controlling these characters are referred

to as sex-linked genes and the inheritance of thesegenes is called sex-linkage or more commonly sex-linked inheritance.

SEX-Linked inheritance of Diseases


137/148

Certain genes which occur on the X

chromosome are more likely to affect a malethan a female.

This is clearly evident in the case of a certain

form of colour blindness and haemophilia,

two important sex-linked inherited diseases

of human.

l k d h d h d


138/148

It is a sex-linked inherited human disease.

People suffering from this disease are not able todistinguish between the colour red and green.

The gene for red-green colour blindness is carriedon the X-chromosome.

Normal vision is dominant over red-green colourblindness.

the condition was often called daltonism, although

this term is now used for a type of colour blindnesscalled deuteranopia.


139/148


140/148

Haemophilia

Haemophilia is an another X linked inherited disease


141/148

Haemophilia is an another X-linked inherited disease.

The people suffering from haemophilia are unable tosynthesize a normal blood protein called Anti-Haemophilic Globulin (AHG) that helps in clotting.

As a result, haemophiliacs can bleed for a long time

even from a very small cut. That is why haemophilia is also called Bleeders

disease.

The gene causing Haemophilia is also a recessive genelocated on the X chromosome similar to that of red-green color blindness.

Haemophilia chart


142/148

Haemophilia chart


143/148

Other X-linked Factors


144/148

It include Brown Enamel on teeth.

Sexual characters like beard and muscularphysique in men, mammary glands, wider pelvis inwomen are not due to sex-linked genes.

Rather they are different expressions of the samegenes.

It is due to the presence of the X or Y chromosome

that creates an environment of maleness orfemaleness causing to the sex chromosomespresent in an individual.

A very few abnormalities are thought to be linkedh h b h


145/148

to the Y chromosome but now even they are open

to more research. It should be remembered that the X-chromosome

of a man always comes from his mother while a

girl inherits one X-chromosome each from hermother and father.

Thus, a man can never transfer a sex-linked gene

directly to his son because the son inherits onlythe Y-chromosome from his father.

Father of Genetics: Gregor Mendel


146/148

at e o Ge et cs: G ego e de

Mendels law: 1). Law of Dominance 2). Law of

segregation 3). Law of independent assortment.

According to Mendel each characters has two alleles:

Dominant and Recessive

Law of Segregation: it states that when a pair of

contrasting alleles come together in an organism they

remain pure and separate again in the gametes.

Law of independent assortment: it states that whenthe parents differ in two or more alleles then

inheritance of a pair of allele is independent of others.

There are exceptions to Mendels law e.g.

Incomplete Dominance, linkage, Multiple

Allelism and Gene Interactions


147/148

Allelism and Gene Interactions.

Mendels law are important because genotypesand phenotypes of next generation can be

predicted even before cross is made.

Chromosomes common to male and femaleindividuals are called Autosomes.

Chromosomes that determine the sex of a

diploid individual are called sex chromosomes.

In humans, there are 46 chromosomes (23 pairs of

chromosomes: 22 pairs are Autosomes and 23rdpair is Sex chromosome, XY in male and XX in


148/148

p ,female.)

Characters controlled by genes on sexchromosomes are called sex-linked characters.

Two important sex-linked inherited human

diseases are haemophilia and colour blindness. They are both due to a recessive gene present on

X-chromosome.

X-chromosome linked recessive diseases are lesscommon in female because of their heterozygosity.

1. Cell Division--2.Structure of Chromosomes--3.Principles of Genetics

Documents

Transcript of 1. Cell Division--2.Structure of Chromosomes--3.Principles of Genetics