How Cells ReproduceChapter 7

Types of Cell Division

Eukaryotic organisms

– Mitosis

– Meiosis

Prokaryotic organisms

– Prokaryotic fission

Roles of Mitosis

• Multicelled organisms

– Growth

– Cell replacement

• Some protistans, fungi, plants, animals

– Asexual reproduction

Chromosome

• A DNA molecule and attached proteins

• Duplicated in preparation for cell division

Organization of Chromosomes

DNA wound tightly around histones before replication

The Cell Cycle

Interphase

• Usually longest part of the cycle (varies by cell type)

• G1 phase normal cell function and growth

• S phase DNA is replicated

• G2 phase proteins made for division

Prophase Metaphase Anaphase Telophase

PMAT

Mitosis

• Period of nuclear division

• Usually followed by cytoplasmic division

Control of the Cycle

• Once S begins, the cycle usually runs through

G2 and mitosis

• The cycle has a built-in molecular brake in G1

• Cancer involves a loss of control over the

cycle, malfunction of the “brakes”

Chromosome Number • Sum total of chromosomes in a cell

• Somatic cells– Chromosome number is diploid (2n)– Two of each type of chromosome– One from mom and one from dad

• Gametes– Chromosome number is haploid (n)– One of each chromosome type

Why are haploid cells needed for gametes?

Human Chromosome Number

• Diploid chromosome number (n) = 46

• Two sets of 23 chromosomes each– One set from father– One set from mother

• Mitosis produces cells with 46 (diploid) chromosomes: two of each type

The Spindle Apparatus• Consists of two distinct sets of microtubules

– Each set extends from one of the cell poles– Two sets overlap at spindle equator– Begins in centrosome (has centrioles in animals)

• Moves chromosomes during mitosis

Maintaining Chromosome

Number

chromosomes (unduplicated) in parent cell at interphase

Stages of Mitosis

Prophase (early, late)

Metaphase

Anaphase

Telophase

Early Prophase: Mitosis Begins

Duplicated chromosomes begin to condense

Late Prophase

• New microtubules are assembled

• One centriole pair is moved toward opposite pole of spindle

• Nuclear envelope starts to break up

Transition to Metaphase

• Spindle forms

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

Metaphase

• All chromosomes are lined up at the spindle equator

• Chromosomes are maximally condensed

Anaphase

• Sister chromatids of each chromosome are pulled apart

• Once separated, each chromatid is a chromosome

Telophase

• Chromosomes decondense

• 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 are in unduplicated form

Cytoplasmic Division

• Usually occurs between late anaphase and end of telophase

• Two mechanisms – Cell plate formation (plants)– Cleavage (animals)

Cell Plate Formation

Animal Cell Division

Actin and myosin filaments pull in membrane and pinch off

Asexual Reproduction

• Single parent produces offspring

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

• Parthenogenesis

Sexual Reproduction

• Involves

– Meiosis

– Gamete production

– Fertilization

• Produces genetic variation among offspring

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

Chromosome From Dad

Gene for eye color

Genes and alleles

Chromosome From Mom

An allele is a different expression of the same gene

Karyotype

Sexual Reproduction Shuffles Alleles

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

• This variation in traits is the basis for evolutionary change

• “Better” traits should lead to more offspring which over time will change the gene pool

Gamete Formation

• Gametes are sex cells (sperm, eggs)

• Arise from germ cells in reproductive organs

testes

ovaries

Chromosome Number• Sum total of chromosomes in a cell

• Germ cells are diploid (2n) (both sets

of chromosomes)

• Gametes are haploid (n) (only one

set of chromosomes)

• Meiosis halves chromosome number

Meiosis: Two Divisions

• Two consecutive nuclear divisions – Meiosis I

– Meiosis II

• DNA is not duplicated between divisions

• Four haploid nuclei form from one diploid

cell

Prophase I

• Each duplicated chromosome pairs with homologue forming a tetrad

• Crossing over --Homologues swap segments

• Each chromosome becomes attached to spindle

Crossing Over• Each chromosome

becomes zippered to its homologue

• All four chromatids are closely aligned

• Nonsister chromosomes exchange segments

Effect of Crossing Over

• After crossing over, each chromosome

contains both maternal and paternal

segments

• Creates new allele combinations in

offspring

Random Alignment

• During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes

• Initial contacts between microtubules and chromosomes are random

Possible Chromosome

Combinations

Y chromosomes rarely cross over with X

You can trace a lineage of males quite readily

East Asian men:1.5 million descendents today

16 million descendents today

Giocangga

Genghis Khan

Metaphase I

• Chromosomes are pushed and pulled into the middle of cell

• The spindle is fully formed

Anaphase I

• Homologous chromosomes segregate

• The sister chromatids remain attached

Telophase I

• The chromosomes arrive at opposite poles

• Usually followed by cytoplasmic division

• Are these cells haploid or diploid?

Prophase II

• Microtubules attach to the kinetochores of the duplicated chromosomes

Metaphase II

• Duplicated chromosomes line up at the spindle equator, midway between the poles

Anaphase II

• Sister chromatids separate to become independent chromosomes

Telophase II

• The chromosomes arrive at opposite ends of the cell

• A nuclear envelope forms around each set of chromosomes

• Four haploid cells, genetically different

Spermatogenesis

GrowthMeiosis I,

Cytoplasmic DivisionMeiosis II,

Cytoplasmic Division

cell differentiation, sperm formation

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

spermato-gonium (diploid )

sperm (mature, haploid male

gametes)

Spermatogenesis

Spermatogenesis

Develop from the spermatogonium germ cell (2n)

Meiosis happens as they migrate through the testes

Produces four viable sperm

OogenesisGrowth

Meiosis I,Cytoplasmic Division

Meiosis II,Cytoplasmic Division

ovum (haploid)

primary oocyte (diploid)

oogonium (diploid)

secondary oocyte

(haploid)

first polar body

(haploid)

three polar bodies

(haploid)

OogenesisOogonium give rise to primary oocyte during embryonic development

Primary oocyte remains in prophase I until activated (one at a time) during puberty by hormones

Hormones start meiosis until metaphase II

Meiosis only complete when contact with sperm is made

Oogenesis

When primary oocyte divides one secondary oocyte receives most of the cytoplasm

Similarly, when the secondary oocyte divides the ovum gets most cytoplasm

Polar bodies degenerate

Oogenesis vs spermatogenesis

1. One gamete (egg) vs four gametes (sperm)

2. At birth female has a set number of primary oocytes

3. Oogenesis is interrupted with resting stages while spermatogenesis is continuous

Fertilization

• Male and female gametes unite

and nuclei fuse

• Fusion of two haploid nuclei produces diploid

nucleus in the zygote

• Which of the two gametes unite is random– Adds to variation among offspring

Factors Contributing to Variation among Offspring

• Crossing over during prophase I

• Random alignment of chromosomes at metaphase I

• Random combination of gametes at fertilization

Mitosis

• Functions– Asexual reproduction– Growth, repair

• Occurs insomatic cells

• Produces clones

Mitosis & Meiosis Compared

Meiosis

• Function – Sexual reproduction

• Occurs in germ cells

• Produces variable offspring

Animal Life Cycle

Usually pretty rapid from meiosis to fertilization

Plant Life Cycle

Not as rapid, from spores

Spores not a fully developed gamete

Can last for many years