Honors BiologyChapter 9

JUST THE FACTS!

Cell Reproduction = the process by which cells divide to create new cells = cell division

Why Most Cells Divide:

• 1. Surface Area to Volume Ratio:- As cells grow their volume increases

faster than their surface area.- This decreases the surface area to volume

ratio.- So, diffusion and other forms of transport

cannot happen fast enough to keep the cell alive.

What would the volume of a cell be if each side measured 4 cm?

Why Most Cells Divide: (cont.)

• 2. DNA Overload:– Genetic information cannot be processed

fast enough in a really large cell to keep the cell functioning.

– Giant amebas have 2 nuclei with 2 sets of chromosomes to overcome this problem!

Cell Cycle = the orderly sequence of growth and division for a cell• Events of the Cell Cycle:

• G1 Phase: Period of growth before DNA is duplicated

• S Phase: DNA is replicated (duplicated)

Cell Cycle

• Events of the Cell Cycle: (Cont.)

• G2 Phase: More growth, and organelles and materials needed for cell division are made.

• M Phase: Cell division – 2 Parts:– Nuclear Division (Mitosis) – Cytoplasm Division (Cytokinesis)

Cell Cycle

B. The Cell Cycle- Series of events that cells go through as they grow and divide

1. Consists of 4 phases (interphase= G1, S, and G2) a. M phase (mitosis)-

division of cell nucleus and cytokinesis

b. G1 phase (gap)-periods of growth and activityc. S phase (synthesis)- DNA synthesized (duplicated)

d. G2 phase (gap)- period of growth and acitivity. Organelles produced.

Two Types of Nuclear Division

TYPE WHO?

(Type of Cell)

WHY?

(Purpose)

Mitosis

Somatic (Body) Cells = any cell but a sex cell

Make identical copies of the original cell for growth, repair and maintenance = clones.

Meiosis

Sexual reproductive cells (sex cells)

To reduce the chromosome number by half.

To increase genetic variation

D. Cytokinesis- division of cytoplasm and organelles. Follows mitosis or meiosis (division of the nucleus)

1. Animal cells- cell membrane drawn inward until cytoplasm is pinched into two nearly equal parts

2. Plant cells- cell plate forms midway between divided nuclei. Gradually develops into separating membrane. Eventually cell wall begins to appear.

Asexual Reproduction

• Single parent produces offspring

• All offspring are genetically identical to one another and to parent

Chromosome = thin thread of DNA wrapped around histone proteins. • Structure:

•

Chromosome

A. Chromosomes- genetic information carried on chromosomes

1. Before cell division each chromosome is replicated (copied)

2. Each chromosome consists of two identical “sister” chromatids

3. Each pair of chromosomes attached to area called centromere

Chromosome Number

• Chromosome number is expressed as either:

• 1. Diploid = 2n = the number of chromosomes found in somatic cells, where n = the number of homologous pairs.

Chromosome Number (cont.)

• Haploid Number = n = the number of chromosomes in a gamete of that organism. (Half the diploid number)

• Examples:– Humans: n = 23– Gorillas: n = 24– Pea Plants: n = 7– Why is it necessary for gametes to have half

the chromosome number of somatic cells?

CHROMOSOMES

• Homologous chromosomes = a matched set; each chromosome has a partner chromosome that looks just like it and contains genes for the same things.

Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome

• One chromosome in each pair from mother, other from

father

• Paternal and maternal chromosomes carry different

alleles (allele = different form of a gene, but for the same

inherited trait. Ex. We have genes for eye color, but you

could have an allele for brown eyes, or an allele for blue

eyes.

Mitosis Details: Interphase

• Usually longest part of the cycle

• Cell increases in mass

• Number of cytoplasmic components doubles

• DNA is duplicated

Mitosis Details: Prophase

Duplicated chromosomes begin to condense

Mitosis Details: Late Prophase • New microtubules are

assembled

• One centriole pair is moved toward opposite pole of spindle

• Nuclear envelope starts to break up

Mitosis Details: Transition to Metaphase

• Spindle forms

• Spindle microtubules become attached to the two sister chromatids of each chromosome

Mitosis Details: Metaphase

• All chromosomes are lined up at the spindle equator

• Chromosomes are maximally condensed

Mitosis Details: Anaphase

• Sister chromatids of each chromosome are pulled apart

• Once separated, each chromatid is a chromosome

Telophase• Chromosomes de-condense

• Two nuclear membranes form, one around each set of unduplicated chromosomes

Results of Mitosis

• Two daughter nuclei • Each with same

chromosome number as parent cell

• Chromosomes in unduplicated form

Mitosis Details - Result

• Mitosis is then followed by Cytokinesis.

• Final Result:– 2 identical offspring cells that have the diploid

chromosome number and the same genetic information as the original parent cell.

Mitosis Details (cont.)

• Abnormalities:– Cancer: uncontrolled cell division

Meiosis Details

Chapter 9

Sexual Reproduction

• Involves

–Meiosis

–Gamete production

–Fertilization• Produces genetic variation among

offspring

Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits

• This variation in traits is the basis for evolutionary change

Factors Contributing to Variation Among Offspring

• Crossing over during prophase I

• Random alignment of chromosomes

at metaphase I

• Random combination of gametes at

fertilization

Gamete Formation

• Gametes are sex cells (sperm, eggs)

• Arise from germ cells

testes

ovaries

anther ovary

Meiosis: Phases and Events• Two consecutive nuclear divisions

–Meiosis I

–Meiosis II

• DNA is NOT duplicated between

divisions

• Four haploid nuclei are formed

Stages of Meiosis

Meiosis I

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

Meiosis II

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

Prophase I• Each duplicated, condensed

chromosome pairs with its homologue• Homologues swap segments =

Crossing Over• Each chromosome becomes attached

to microtubules of newly forming spindle

Crossing Over

•Each chromosome

becomes zippered to its

homologue

•All four chromatids are

closely aligned = tetrad

•Non-sister chromosomes

exchange segments

Meiosis I

Each homologue in the cell pairs with its partner,

then the partners separate

Effect of Crossing Over

• After crossing over, each chromosome

contains both maternal and paternal

segments

• Creates new allele combinations in

offspring

Metaphase I• Chromosomes are pushed and

pulled into the middle of cell• Sister chromatids of one homologue

orient toward one pole, and those of other homologue toward opposite pole

• The spindle is now fully formed

Anaphase I

• Homologous chromosomes

segregate(separate) from each

other

• The sister chromatids of each

chromosome remain attached

Telophase I & Cytokinesis

• The chromosomes arrive at opposite

poles

• The cytoplasm divides

• There are now two haploid cells

• This completes Meiosis I

Meiosis II - Stages

Prophase II Metaphase II Anaphase II Telophase II

Prophase II

• Microtubules attach to the kinetochores of the duplicated chromosomes

• Motor proteins drive the movement of chromosomes toward the spindle’s equator

Metaphase II

• All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles

Anaphase II

• Sister chromatids separate to become independent chromosomes

• Motor proteins interact with microtubules to move the separated chromosomes to opposite poles

Telophase II & Cytokinesis

• The chromosomes arrive at opposite ends of the cell

• A nuclear envelope forms around each set of chromosomes

• The cytoplasm divides

• There are now four haploid cells

Random Alignment

• Either the maternal or paternal member of a homologous pair can end up at either pole

• The chromosomes in a gamete are a mix of chromosomes from the two parents

Possible Chromosome Combinations

As a result of random alignment, the number

of possible combinations of chromosomes in a gamete is:

2n

(Humans = 8,388,608!!!)

(n is number of chromosome types)

Possible Chromosome

Combinations

or

or

or

1 2 3

Results of Meiosis

• Meiosis

– Four haploid cells produced

– Differ from parent and one another

Abnormalities

• Nondisjunction: Chromosome pairs fail to separate during Meiosis

• Causes Chromosomal abnormalities – extra or missing chromosmes

• Ex. Down Syndrome

Spermatogenesis

GrowthMitosis I,

Cytoplasmic divisionMeiosis II,

Cytoplasmic division

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

spermato-gonium

(diploid male reproductive

cell)

Oogenesis

Growth Mitosis I,Cytoplasmic division

Meiosis II,Cytoplasmic division

ovum (haploid)

primary oocyte (diploid)

oogonium (diploid

reproductive cell) secondary

oocyte haploid)

first polar body

haploid)

three polar bodies

haploid)

Fertilization

• Male and female gametes unite and nuclei

fuse

• Fusion of two haploid nuclei produces

diploid nucleus in the zygote

• Which two gametes unite is random

– Adds to variation among offspring

Mitosis• Functions

– Asexual reproduction– Growth, repair

• Occurs in somatic cells

• Produces clones

Mitosis & Meiosis Compared

Meiosis• Function

– Sexual reproduction

• Occurs in germ cells

• Produces variable offspring