Chapter 19 & 20 Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on...

Chapter 19 & 20

Biology 25: Human BiologyBiology 25: Human Biology

Prof. GonsalvesProf. Gonsalves

Los Angeles City CollegeLos Angeles City College

Based on Mader’s Based on Mader’s Human BiologyHuman Biology,7,7thth edition and Fox’s 8edition and Fox’s 8thth ed ed

PowerpointsPowerpoints

HeredityHeredity: The transmission of traits from : The transmission of traits from one generation to another.one generation to another.

VariationVariation: Offspring are different from : Offspring are different from their parents and siblings.their parents and siblings.

GeneticsGenetics: The scientific study of heredity : The scientific study of heredity and hereditary variation. and hereditary variation. Involves study of cells, individuals, their Involves study of cells, individuals, their offspring, and populations.offspring, and populations.

I. History of GeneticsI. History of Genetics

Blending Hypothesis:Blending Hypothesis: In 1800s biologists and plant breeders In 1800s biologists and plant breeders

suggested that traits of parents mix to form suggested that traits of parents mix to form intermediateintermediate traits in traits in

offspring.offspring.

ParentsParents OffspringOffspring

Red flower x White flowerRed flower x White flower Pink flowerPink flower

Tall height x Short heightTall height x Short height Medium heightMedium height

Blue bird x Yellow birdBlue bird x Yellow bird Green birdsGreen birds

Fair skin x dark skinFair skin x dark skin Medium skin colorMedium skin color

If If blending blending always occurred, eventually all always occurred, eventually all extremeextreme characteristicscharacteristics

would would disappeardisappear from the population. from the population.

Gregor Mendel:Gregor Mendel: Established genetics as a science in 1860s. Established genetics as a science in 1860s.

Considered the founder of Considered the founder of modern geneticsmodern genetics..

II. Modern GeneticsII. Modern Genetics

Began as a science in 1860s. Began as a science in 1860s.

Gregor Mendel:Gregor Mendel: An Austrian monk, who was a farmer’s An Austrian monk, who was a farmer’s

son. He was trained in mathematics, chemistry, and physics.son. He was trained in mathematics, chemistry, and physics.

Studied the breeding patterns of plants for over 10 years.Studied the breeding patterns of plants for over 10 years.

Artificially crossed Artificially crossed peaspeas, watermelons, and other plants., watermelons, and other plants.

Kept Kept meticulous recordsmeticulous records of thousands of breedings and of thousands of breedings and

resulting offspring.resulting offspring.

Rejected blending hypothesisRejected blending hypothesis, and stressed that heritable , and stressed that heritable

factors (factors (genesgenes) retain their ) retain their individualityindividuality generation after generation after

generation.generation.

II. Modern GeneticsII. Modern Genetics

Gregor Mendel:Gregor Mendel:

Calculated the Calculated the mathematical probabilitiesmathematical probabilities of of

inheriting many genetic traits.inheriting many genetic traits.

Published results in 1866. They were Published results in 1866. They were largely ignoredlargely ignored

due to fervor surrounding Darwin’s publications on due to fervor surrounding Darwin’s publications on

evolution.evolution.

Discouraged by the lack of attention from the Discouraged by the lack of attention from the

scientific community, he quit his work and died a few scientific community, he quit his work and died a few

years later.years later.

Importance of Mendel’s work was not appreciated Importance of Mendel’s work was not appreciated

until until early 1900searly 1900s when his paper was rediscovered. when his paper was rediscovered.

III. Mendel’s ExperimentsIII. Mendel’s Experiments

Used “Used “true-breedingtrue-breeding” or ” or purebredpurebred plant varieties for seven plant varieties for seven

pea characteristics. Self-pollination produces pea characteristics. Self-pollination produces all identical all identical

offspring.offspring.

Using Using artificial pollinationartificial pollination, he crossed true-bred varieties., he crossed true-bred varieties.

Trait Trait VarietiesVarieties

Flower colorFlower color Purple or whitePurple or white

Seed colorSeed color Yellow Yellow or or greengreen

Seed shapeSeed shape Round or wrinkledRound or wrinkled

Pod colorPod color GreenGreen or or YellowYellow

Pod shapePod shape Smooth or constrictedSmooth or constricted

Flower positionFlower position Axial or terminalAxial or terminal

Plant heightPlant height Tall or shortTall or short

III. Summary of Mendel’s ResultsIII. Summary of Mendel’s Results

All plants displayed All plants displayed one trait only.one trait only.

Trait Trait VarietiesVarieties OffspringOffspring

Flower colorFlower color Purple or whitePurple or white 100% Purple100% Purple

Seed colorSeed color Yellow Yellow or or greengreen 100% Yellow100% Yellow

Seed shapeSeed shape Round or wrinkledRound or wrinkled 100% Round100% Round

Pod colorPod color GreenGreen or or YellowYellow 100% Green100% Green

Pod shapePod shape Smooth or constrictedSmooth or constricted 100% Smooth100% Smooth

Flower positionFlower position Axial or terminalAxial or terminal 100% Axial100% Axial

Plant heightPlant height Tall or shortTall or short 100% Tall100% Tall

The trait that prevailed was The trait that prevailed was dominantdominant, the other , the other recessiverecessive..

IV. Mendel’s ConclusionsIV. Mendel’s Conclusions

1.1. Results indicate that blending Results indicate that blending

hypothesis is not true.hypothesis is not true.

2. 2. Only one of the two traits appeared Only one of the two traits appeared

in the first generation. He called this in the first generation. He called this

the the dominantdominant trait. He called the trait. He called the

trait that disappeared the trait that disappeared the recessiverecessive

trait.trait.


1.1. Results indicate that the Results indicate that the recessive trait is recessive trait is

intactintact. .

2. 2. The crossbred plants with purple flowers The crossbred plants with purple flowers

must be must be carrying the genetic informationcarrying the genetic information to to

produce white flowers. produce white flowers.

3.3. The crossbred plants with purple flowers The crossbred plants with purple flowers

are are genetically differentgenetically different from the purebred from the purebred

plants, even though they look the same.plants, even though they look the same.


4.4. Must distinguish between: Must distinguish between:

PhenotypePhenotype:: Physical appearancePhysical appearance of individual. of individual.

Example:Example: Two phenotypesTwo phenotypes for flower color. for flower color.

Purple flowersPurple flowers

White flowers.White flowers.

Genotype:Genotype: Genetic makeupGenetic makeup of an individual. of an individual.

Not all purple flowers are genetically identical.Not all purple flowers are genetically identical.


5.5. Each individual carries Each individual carries two genestwo genes for a given for a given

genetic trait. One gene comes from the genetic trait. One gene comes from the

individual’s mother, the other from the father.individual’s mother, the other from the father.

There are two There are two alternative forms of genesalternative forms of genes or or

hereditary units. hereditary units.

The alternative forms of these hereditary units are The alternative forms of these hereditary units are

called called allelesalleles. .

P: Allele for purple flowersP: Allele for purple flowers

p: Allele for white flowersp: Allele for white flowers


6. 6. In a given individual, the two genes for a In a given individual, the two genes for a

given trait may be the same given trait may be the same alleleallele (form of a (form of a

gene) or different.gene) or different.

PhenotypePhenotype Genotype:Genotype:

PurplePurple PP (Homozygous dominant)PP (Homozygous dominant)

PurplePurple Pp (Heterozygous dominant)Pp (Heterozygous dominant)

WhiteWhite pp (Homozygous recessive)pp (Homozygous recessive)

Homologous Chromosomes Bear the Two Alleles for Each Characteristic

Phenotype and Genotype of Mendel’s Pea Plants

Punnet SquarePunnet Square::

Used to determine the outcome of a cross between Used to determine the outcome of a cross between two individuals.two individuals.

Heterozygotes make 1/2 P and 1/2 p gametes.Heterozygotes make 1/2 P and 1/2 p gametes.

PP p p

PP PP PP Pp Pp

pp PpPp pp pp

Offspring:Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp Phenotype: 3/4 Purple and 1/4 white

Genotypic and Phenotypic Ratios of F2 Generation

VI. Principles of Mendelian GeneticsVI. Principles of Mendelian Genetics

1. 1. There are alternative forms of There are alternative forms of genesgenes, the units that , the units that

determine heritable traits. determine heritable traits.

These alternative forms are called These alternative forms are called allelesalleles..

Example:Example:

Pea plants have one Pea plants have one alleleallele for purple flower color, for purple flower color,

and another for white color.and another for white color.


2. 2. For each inherited characteristic, an individual For each inherited characteristic, an individual

has has two genestwo genes: one from each parent. : one from each parent.

In a given individual, the genes may be the same In a given individual, the genes may be the same

allele (allele (homozygoushomozygous) or they may be different ) or they may be different

alleles (alleles (heterozygousheterozygous).).


3. 3. When two genes of a pair are different alleles, only one is When two genes of a pair are different alleles, only one is

fully expressed (fully expressed (dominant alleledominant allele). The other allele has no ). The other allele has no

noticeable effect on the organism’s appearance (noticeable effect on the organism’s appearance (recessive recessive

alleleallele).).

ExampleExample::

Purple allele for flower color is Purple allele for flower color is dominantdominant

White allele for flower color is White allele for flower color is recessiverecessive


4. 4. A sperm or egg cell (gamete) A sperm or egg cell (gamete) only contains one alleleonly contains one allele or gene or gene

for each inherited trait. for each inherited trait.

Principle of SegregationPrinciple of Segregation: Alleles : Alleles segregatesegregate (separate) during (separate) during

gamete formation.gamete formation.

(When? During meiosis I)(When? During meiosis I)

During During fertilizationfertilization, sperm and egg each contribute one allele , sperm and egg each contribute one allele

to the new organism, restoring the allele pair.to the new organism, restoring the allele pair.


5. 5. Principle of Independent AssortmentPrinciple of Independent Assortment: :

Two different genetic characteristics are Two different genetic characteristics are

inherited inherited independentlyindependently of each other.* of each other.*

*As long as they are on different chromosomes.*As long as they are on different chromosomes.

Mendel did not know about meiosis, but Mendel did not know about meiosis, but

meiosis explains this observation.meiosis explains this observation.

Why? Why?

How are chromosomes shuffled during How are chromosomes shuffled during

meiosis I?meiosis I?

VII. Human GeneticsVII. Human Genetics

Inheritance of human traits. Inheritance of human traits.

Most genetic diseases are recessive.Most genetic diseases are recessive.

Dominant TraitsDominant Traits Recessive TraitsRecessive Traits

Widow’s peakWidow’s peak Straight hairlineStraight hairline

Freckles Freckles No frecklesNo freckles

Free earlobeFree earlobe Attached earlobeAttached earlobe

NormalNormal Cystic fibrosisCystic fibrosis

NormalNormal PhenylketonuriaPhenylketonuria

NormalNormal Tay-Sachs diseaseTay-Sachs disease

Normal Normal AlbinismAlbinism

Normal hearingNormal hearing Inherited deafnessInherited deafness

Huntington’s DiseaseHuntington’s Disease NormalNormal

DwarfismDwarfism Normal heightNormal height

Eucaryotic cell division is a more complex and time consuming process than binary fissionEucaryotic cell division is a more complex and time consuming process than binary fission

Features of Eucaryotic DNAFeatures of Eucaryotic DNA

1. DNA is in 1. DNA is in multiplemultiple linear linear chromosomeschromosomes.. Unique number for each species:Unique number for each species:

• Humans have 46 chromosomes.Humans have 46 chromosomes.• Cabbage has 20, mosquito 6, and fern over 1000.Cabbage has 20, mosquito 6, and fern over 1000.

2. Large Genome: Up to 3 billion base pairs (humans) 2. Large Genome: Up to 3 billion base pairs (humans) Contains up to 50,000-150,000 genesContains up to 50,000-150,000 genes Human genome projectHuman genome project is determining the sequence of entire human DNA. is determining the sequence of entire human DNA.

3. DNA is enclosed by 3. DNA is enclosed by nuclear membranenuclear membrane..Correct distribution of Correct distribution of multiplemultiple chromosomes in each daughter cell requires a much chromosomes in each daughter cell requires a much more elaborate process than binary fission.more elaborate process than binary fission.

ChromosomesChromosomes ChromatinChromatin

Tightly packaged DNATightly packaged DNA Unwound DNAUnwound DNA

Found only during Found only during cell cell FoundFound throughout cell throughout cell divisiondivision cyclecycle

DNA is DNA is notnot being used being used DNA is being used DNA is being used

for macromolecule for macromolecule for macromolecule for macromolecule

synthesis.synthesis. synthesis. synthesis.

DNA: Found as Chromosomes or Chromatin

Eucaryotic Chromosomes Duplicate Before Each Cell Division

Cell Cycle of Eucaryotic Cells

Sequence of events from the time a cell is formed, until the cell divides Sequence of events from the time a cell is formed, until the cell divides once again.once again.

Before cell division, the cell must:Before cell division, the cell must: Precisely copy genetic material (DNA)Precisely copy genetic material (DNA) Roughly double its cytoplasmRoughly double its cytoplasm Synthesize organelles, membranes, proteins, and other molecules.Synthesize organelles, membranes, proteins, and other molecules.

Cell cycle is divided into two main phases:Cell cycle is divided into two main phases: InterphaseInterphase: Stage between cell divisions: Stage between cell divisions Mitotic PhaseMitotic Phase: Stage when cell is dividing: Stage when cell is dividing

Eucaryotic Cell Cycle: Interphase + Mitotic Phase

Mitosis:Mitosis: The Stages of Cell Division The Stages of Cell Division

1. Prophase1. Prophase

Chromatin condenses into Chromatin condenses into chromosomeschromosomes, which appear as two , which appear as two

sister chromatids joined by a sister chromatids joined by a centromerecentromere..

Nucleoli disappear.Nucleoli disappear.

Nuclear envelope breaks apart. Nuclear envelope breaks apart.

In animal cells, In animal cells, mitotic spindlemitotic spindle begins to form as begins to form as mictotubulesmictotubules

grow out of two grow out of two centrosomescentrosomes or or microtubulemicrotubule organizingorganizing centerscenters

(MTOCs).(MTOCs).

• Each centrosome is made up of a pair of Each centrosome is made up of a pair of centriolescentrioles. .

Microtubules attach to Microtubules attach to kinetochoreskinetochores on chromatids and begin to on chromatids and begin to

move chromosomes towards center of cell.move chromosomes towards center of cell.

Centrosomes begin migrating to opposite poles of cell.Centrosomes begin migrating to opposite poles of cell.

Interphase and Prophase of Mitosis in Animal Cell


2. Metaphase2. Metaphase

Short period in which chromosomes line up along Short period in which chromosomes line up along

equatorial plane of cell (equatorial plane of cell (metaphase platemetaphase plate).).

Chromosomes are completely condensed and easy to Chromosomes are completely condensed and easy to

visualize.visualize.

Mitotic spindle is fully formed.Mitotic spindle is fully formed.

Kinetochores of sister chromatids face opposite sides Kinetochores of sister chromatids face opposite sides

and are attached to spindle microtubules at opposite and are attached to spindle microtubules at opposite

ends of the cell.ends of the cell.

Metaphase, Anaphase, and Telophase of Mitosis in an Animal Cell


3.Anaphase3.Anaphase

Centromeres of sister chromatids begin to Centromeres of sister chromatids begin to separateseparate..

Each chromatid is now an Each chromatid is now an independent daughter independent daughter

chromosomechromosome..

The separate chromosomes are pulled toward opposite The separate chromosomes are pulled toward opposite

ends by spindle microtubules, attached to the ends by spindle microtubules, attached to the

kinetochores.kinetochores.

Cell elongates as poles move farther apart.Cell elongates as poles move farther apart.

Anaphase ends when a complete set of chromosomes Anaphase ends when a complete set of chromosomes

reaches each pole.reaches each pole.


4. Telophase4. Telophase

Cell continues to elongate.Cell continues to elongate.

Cell returns to interphase conditions:Cell returns to interphase conditions:

• A nuclear envelope forms around each set of A nuclear envelope forms around each set of

chromosomes.chromosomes.

• Chromosomes uncoil, becoming chromatin Chromosomes uncoil, becoming chromatin

threads.threads.

• Nucleoli reappear.Nucleoli reappear.

• Spindle microtubules disappear.Spindle microtubules disappear.

CytokinesisCytokinesis usually occurs at the end of this stage usually occurs at the end of this stage

Mitotic PhaseMitotic Phase: Mitosis + Cytokinesis: Mitosis + Cytokinesis

CytokinesisCytokinesis

The division of cytoplasm to produce two daughter The division of cytoplasm to produce two daughter

cells. Usually begins during cells. Usually begins during telophasetelophase..

• In In animal cellsanimal cells: Division is accomplished by a : Division is accomplished by a

cleavage furrowcleavage furrow that encircles the cell like a ring that encircles the cell like a ring

in the equator region.in the equator region.

• In In plant cellsplant cells: Division is accomplished by the : Division is accomplished by the

formation of a formation of a cell platecell plate between the daughter between the daughter

cells. Each cell produces a plasma membrane and cells. Each cell produces a plasma membrane and

a cell wall on its side of the plate.a cell wall on its side of the plate.

Cytokinesis in Animal and Plant Cells

Animal Cell Plant Cell

External Factors Control MitosisExternal Factors Control Mitosis

1. Anchorage1. Anchorage Most cells cannot divide unless they are attached to a Most cells cannot divide unless they are attached to a

solid surface.solid surface. May prevent inappropriate growth of detached cellsMay prevent inappropriate growth of detached cells

2. Nutrients and growth factors2. Nutrients and growth factors

Lack of nutrients can limit mitosisLack of nutrients can limit mitosis Growth factorsGrowth factors: Proteins that stimulate cell division.: Proteins that stimulate cell division.

3. Cell density3. Cell density

Density-dependent inhibitionDensity-dependent inhibition:: Cultured cells will stop Cultured cells will stop dividing after a single layer covers the petri dish. dividing after a single layer covers the petri dish. Mitosis is inhibited by high cell density.Mitosis is inhibited by high cell density.

Cancer cellsCancer cells do not demonstrate density inhibition do not demonstrate density inhibition

Density Dependent Inhibition of Mitosis

Normal Cells Stop Dividing at High Cell DensityCancer Cells are Not Inhibited by High Cell Density

Cell-Cycle Control SystemThere are three critical points at which the cell cycle is controlledThere are three critical points at which the cell cycle is controlled**::1. G1 Checkpoint1. G1 Checkpoint: Prevents cell from entering S phase and duplicating : Prevents cell from entering S phase and duplicating

DNA.DNA. Most important checkpoint.Most important checkpoint. Amitotic cells (muscle and nerve cells) are frozen here.Amitotic cells (muscle and nerve cells) are frozen here.

2. G2 Checkpoint:2. G2 Checkpoint: Prevents cell from entering mitosis. Prevents cell from entering mitosis.3. M Checkpoint:3. M Checkpoint: Prevents cell from entering cytokinesis. Prevents cell from entering cytokinesis.

*Cells must have proper growth factors to get through each checkpoint.*Cells must have proper growth factors to get through each checkpoint.

Cell Division is Controlled at Three Key Stages

Growth factors arerequired to passeach checkpoint

Cancer is a Disease of the Cell Cycle Cancer kills 1 in 5 people in the United States.Cancer kills 1 in 5 people in the United States. Cancer cells divide excessively and invade other body tissues.Cancer cells divide excessively and invade other body tissues. Tumor: Tumor: Abnormal mass of cells that originates from uncontrolled mitosis of a single Abnormal mass of cells that originates from uncontrolled mitosis of a single

cell.cell. Benign tumor:Benign tumor: Cancer cells remain in original site. Can easily be removed or treatedCancer cells remain in original site. Can easily be removed or treated

Malignant tumor:Malignant tumor: Cancer cells have ability to “detach” from tumor and spread to Cancer cells have ability to “detach” from tumor and spread to other organs or tissuesother organs or tissues

Metastasis:Metastasis: Spread of cancer cells form site of origin to another organ or tissue. Spread of cancer cells form site of origin to another organ or tissue. Tumor cells travel through blood vessels or lymph nodes.Tumor cells travel through blood vessels or lymph nodes.

Functions of Mitosis in Eucaryotes: Functions of Mitosis in Eucaryotes:

1.1. Growth:Growth: All All somaticsomatic cells that originate after a new cells that originate after a new individual is created are made by mitosis.individual is created are made by mitosis.

2. 2. Cell replacement:Cell replacement: Cells that are damaged or destroyed Cells that are damaged or destroyed due to disease or injury are replaced through mitosis.due to disease or injury are replaced through mitosis.

3. 3. Asexual Reproduction:Asexual Reproduction: Mitosis is used by organisms Mitosis is used by organisms that reproduce asexually to make offspring.that reproduce asexually to make offspring.

Meiosis: Generates haploid gametesMeiosis: Generates haploid gametes Reduces the number of chromosomes by half, producing Reduces the number of chromosomes by half, producing

haploidhaploid cells from diploid cells. cells from diploid cells. Also produces Also produces genetic variabilitygenetic variability, each gamete is , each gamete is

different, ensuring that two offspring from the same different, ensuring that two offspring from the same parents are never identical.parents are never identical.

Two divisions: Meiosis I and meiosis II. Chromosomes Two divisions: Meiosis I and meiosis II. Chromosomes are duplicated in are duplicated in interphaseinterphase prior to Meiosis I. prior to Meiosis I. Meiosis IMeiosis I:: SeparatesSeparates the members of each the members of each homologous homologous

pair of chromosomespair of chromosomes. . ReductiveReductive division. division. Meiosis IIMeiosis II: : Separates chromatidsSeparates chromatids into individual into individual

chromosomes.chromosomes.

Interphase:Chromosomesreplicate

Meiosis I: Reductive division. Homologouschromosomes separate

Meiosis II: Sister chromatidsseparate

STAGES OF MEIOSIS

Meiosis I: Separation of Homologous ChromosomesMeiosis I: Separation of Homologous Chromosomes1. 1. Prophase I:Prophase I:

Most complex phase of meiosis (90% of time)Most complex phase of meiosis (90% of time) Chromatin condenses into chromosomes. Chromatin condenses into chromosomes. Nuclear membrane and nucleoli disappear.Nuclear membrane and nucleoli disappear. Centrosomes move to opposite poles of cell and Centrosomes move to opposite poles of cell and

microtubules attach to chromatids.microtubules attach to chromatids. Synapsis:Synapsis: Homologous chromosomes pair up and Homologous chromosomes pair up and

form aform a tetradtetrad of 4 sister chromatids.of 4 sister chromatids. Crossing overCrossing over:: DNA is exchanged between DNA is exchanged between

homologous chromosomes, resulting inhomologous chromosomes, resulting in genetic genetic recombinationrecombination. . Unique to meiosisUnique to meiosis. .

ChiasmataChiasmata: : Sites of DNA exchange.Sites of DNA exchange.

Prophase I: Crossing Over Between Homologous Chromosomes

Meiosis I:Meiosis I: Separation of Homologous Separation of Homologous

ChromosomesChromosomes

2. 2. Metaphase I:Metaphase I: Chromosome tetrads (homologous Chromosome tetrads (homologous

chromosomes) line up in the middle of the chromosomes) line up in the middle of the cell.cell.

Each homologous chromosome faces opposite Each homologous chromosome faces opposite poles of the cell.poles of the cell.

Meiosis I: Homologous Chromosomes Separate

Stages of Meiosis: Meiosis IStages of Meiosis: Meiosis I

3. 3. Anaphase I:Anaphase I: Chromosome tetrads split up.Chromosome tetrads split up. Homologous chromosomesHomologous chromosomes of each pairof each pair separateseparate, ,

moving towards opposite poles.moving towards opposite poles. Random assortmentRandom assortment: One chromosome from each : One chromosome from each

homologous pair is shuffled into the two daughter cells, homologous pair is shuffled into the two daughter cells, randomly and independently of the other pairs. randomly and independently of the other pairs.

Random assortmentRandom assortment increases genetic diversityincreases genetic diversity of of offspring. Possible combinations:offspring. Possible combinations: 22nn..

One human cell can generate 2One human cell can generate 22323 or over 8.3 million or over 8.3 million different gametes by random assortment alone.different gametes by random assortment alone.

Random Assortment of Homologous Chromosomes During Meiosis I Generates Many Possible Gametes

Meiosis IMeiosis I: : Separation of Homologous Separation of Homologous

ChromosomesChromosomes

4. 4. Telophase I and Cytokinesis:Telophase I and Cytokinesis: Chromosomes reach opposite Chromosomes reach opposite

poles of the cell.poles of the cell.Nucleoli reorganize, chromosomes Nucleoli reorganize, chromosomes

uncoil, and cytokinesis occurs.uncoil, and cytokinesis occurs.New cells are haploid.New cells are haploid.

Meiosis IIMeiosis II: Separation of Sister Chromatids : Separation of Sister Chromatids

During interphase that follows meiosis I, no DNA During interphase that follows meiosis I, no DNA replication occurs.replication occurs.

Interphase may be very brief or absent.Interphase may be very brief or absent.

Meiosis II is very Meiosis II is very similar to mitosissimilar to mitosis..

1. 1. Prophase II:Prophase II: Very brief, chromosomes reform.Very brief, chromosomes reform.No crossing over or synapsis.No crossing over or synapsis.Spindle forms and starts to move Spindle forms and starts to move

chromosomes towards center of the cell.chromosomes towards center of the cell.


2. 2. Metaphase II:Metaphase II: Very brief, individual chromosomes line up Very brief, individual chromosomes line up

in the middle of the cell.in the middle of the cell.Kinetochores of chromatids face opposite Kinetochores of chromatids face opposite

poles.poles.

3. 3. Anaphase II:Anaphase II: Chromatids separate and move towards Chromatids separate and move towards

opposite ends of the cell.opposite ends of the cell.

Meiosis II: Separation of Sister Chromatids


4. 4. Telophase II:Telophase II: Nuclei form at opposite ends of the Nuclei form at opposite ends of the

cell.cell.Cytokinesis occurs.Cytokinesis occurs.

Product of meiosisProduct of meiosis: :

Four (4) haploid gametes, each Four (4) haploid gametes, each genetically different from the other.genetically different from the other.

Meiosis Produces Four Genetically Different Gametes

Mitosis versus Meiosis (Review)Mitosis versus Meiosis (Review)MitosisMitosis MeiosisMeiosis

OneOne cell divisioncell division TwoTwo successive cell divisionssuccessive cell divisions

Produces Produces twotwo (2) cells(2) cells ProducesProduces four four (4) cells(4) cells

Produces Produces diploiddiploid cellscells ProducesProduces haploidhaploid gametesgametes

Daughter cells are geneticallyDaughter cells are genetically Cells are geneticallyCells are genetically differentdifferent fromfromidenticalidentical to mother cellto mother cell mother cell and each othermother cell and each other

No crossing overNo crossing over Crossing over*Crossing over*

Functions:Functions: Growth, Growth, Functions:Functions: Sexual reproduction Sexual reproductioncell replacement, andcell replacement, andasexual reproductionasexual reproduction

**Crossing overCrossing over: : Exchange of DNA between homologous chromosomesExchange of DNA between homologous chromosomes..

Meiosis in Males and Females Meiosis in Males and Females

Spermatogenesis:Spermatogenesis: Four sperm cells are made.Four sperm cells are made. Starts in puberty and occurs continuously.Starts in puberty and occurs continuously. Males produce millions of sperm cells a month.Males produce millions of sperm cells a month.

OogenesisOogenesis:: Only one large egg is produced. The other three Only one large egg is produced. The other three

cells are small polar bodies. cells are small polar bodies. Oogenesis starts before birth in females, stops at Oogenesis starts before birth in females, stops at

Prophase I, and resumes during puberty.Prophase I, and resumes during puberty. Meiosis is completed only after fertilization.Meiosis is completed only after fertilization. Females make one mature egg/month.Females make one mature egg/month.

Sources of Genetic Variability in Sexual Sources of Genetic Variability in Sexual

ReproductionReproduction

1. Crossing Over1. Crossing Over: After crossing over and synapsis, sister : After crossing over and synapsis, sister chromatids are no longer identical.chromatids are no longer identical.

2. Independent Assortment2. Independent Assortment:: Each human can produce Each human can produce over 8.3 million different gametes by random shuffling over 8.3 million different gametes by random shuffling of chromosomes in meiosis I.of chromosomes in meiosis I.

3. Fertilization3. Fertilization:: A couple can produce over A couple can produce over 64 trillion64 trillion (8.3 (8.3 million x 8.3 million) different zygotes during million x 8.3 million) different zygotes during fertilization. This figure fertilization. This figure does notdoes not take into account take into account diversity created by crossing over.diversity created by crossing over.

Accidents During Meiosis Can Cause Chromosomal Accidents During Meiosis Can Cause Chromosomal

AbnormalitiesAbnormalities NondisjunctionNondisjunction: Chromosomes fail to separate.: Chromosomes fail to separate.

Members of a pair of homologous chromosomes fail Members of a pair of homologous chromosomes fail to separate during meiosis I or:to separate during meiosis I or:

Sister chromatids fail to separate during meiosis II. Sister chromatids fail to separate during meiosis II. Nondisjunction Nondisjunction increasesincreases with with ageage.. Gametes (and zygotes) will have an extra chromosome, Gametes (and zygotes) will have an extra chromosome,

others will be missing a chromosome.others will be missing a chromosome. TrisomyTrisomy: Individuals with one extra chromosome, : Individuals with one extra chromosome,

three instead of pair. Have 47 chromosomes in cells.three instead of pair. Have 47 chromosomes in cells. MonosomyMonosomy: Missing a chromosome, one instead of : Missing a chromosome, one instead of

pair. Have 45 chromosomes in cells.pair. Have 45 chromosomes in cells.

Nondisjunction of Chromosomes During Meiosis Produces Abnormal Gametes

Accidents During Meiosis Can Result in a Trisomy Accidents During Meiosis Can Result in a Trisomy

or Monosomyor Monosomy Most abnormalities in numbers of Most abnormalities in numbers of autosomesautosomes are very are very

serious or fatal.serious or fatal. Down’s syndromeDown’s syndrome: Caused by a : Caused by a trisomytrisomy of of

chromosome number 21 (1 in 700 births). Mental chromosome number 21 (1 in 700 births). Mental retardation, mongoloid features, and heart defects.retardation, mongoloid features, and heart defects.

Most abnormalities of Most abnormalities of sex chromosomessex chromosomes do not affect do not affect survival.survival.

Klinefelter SyndromeKlinefelter Syndrome: Males with an : Males with an extraextra sex sex chromosome (XXY) (1 in 1000 male births).chromosome (XXY) (1 in 1000 male births).

Turner SyndromeTurner Syndrome: Females : Females missingmissing one sex one sex chromosome (XO) (1 in 2500 female births).chromosome (XO) (1 in 2500 female births).

Down’s Syndrome is More Common in Children Born to Older Mothers

Abnormal Numbers of Sex Chromosomes Usually Do Not Affect Survival

Klinefelter Syndrome (XXY) Turner Syndrome (XO)Incidence: 1:1000 male births Incidence: 1 in 2500 female births

Chapter 19 & 20 Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on...

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Transcript of Chapter 19 & 20 Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on...