MeiosisMeiosisCh. 9Ch. 9

GametesGametesGametes are reproductive cells

(eggs and sperm) that contain half the complement of chromosomes found in somatic cells◦the gametes fuse to form a new cell

called a zygote, which contains two complete copies of each chromosome the fusion of gametes is called

fertilization, or syngamy egg + sperm = Zygote

Haploid Cell vs. Diploid Haploid Cell vs. Diploid CellCell

Typically, each cell of your body has 46 chromosomes..23 from each parent

So, you have what we call a Diploid value of 46

Or, referred to as 2N = 46 Your gametes, however,

have 1N values 1N = 23….This is a Haploid

condition All your normal body cells

are diploid, only your gametes are haploid◦ THIS IS TRUE FOR MOST

ORGANISMS!!!!!!

Sex Cells

Chromosome NumberChromosome Number• Every organisms inherits chromosomes from

their parents.• Asexually reproducing organisms pass on

their full set of chromosome creating offspring that are essentially clones of themselves• Sexually reproducing organisms pass on

half of their chromosomes through their sex cells. The offspring created inherits half of its chromosomes from each parent.

Human Life CycleHuman Life Cycle

Gamete- egg and sperm

Zygote- cell produced when egg and sperm fuse

Fertilization/ syngamy- fusion of gametes

TraitsTraits• Inheritable traits are carried on factors called genes and are

passed down from generation to generation.• Genes are located on DNA within the nucleus of the cell.• There are two alleles for every gene. One allele is inherited

from each parent• Alleles are alternate versions of a gene

• The process by which chromosomes are separated in the

production of sex cells is known as Meiosis.

Homologous Homologous ChromosomesChromosomes

• Homologous Chromosomes- chromosomes that carry the alleles for the same gene.• Diploid Cells- (2N)

“Two sets”; have both sets of homologous chromosomes (All body cells are diploid)

• Haploid Cells- (1N) “one set”; has a single set of genes. (All sex cells are haploid)

MeiosisMeiosis

Meiosis- is the process by which the homologous chromosomes are separated in a diploid cell to produce a haploid cell

Meiosis involves two divisions, meiosis I and meiosis II

◦ meiosis I separates the homologues in a homologous pair

◦ meiosis II separates the replicate sister chromatids

Meiosis IMeiosis I Meiosis I is traditionally divided into four

sequential stages

1. Prophase I Homologous chromsomes pair up and exchange

segments

2. Metaphase I The paired homologous chromosomes align on a

central plane

3. Anaphase I Homologues are pulled apart and move to opposite

poles of the cell

4. Telophase I Individual replicated chromosomes gather at each

of the two poles

Prophase IProphase I

Prior to the start of Meiosis, chromosomes are replicated during Interphase.

During Prophase I homologous chromosomes pair up to form a tetrad (there are 4 chromatids in a tetrad)

Crossing over may occur when the tetrads exchange a portion of the chromatids◦ During crossing-over

chromatids break and may be reattached to a different homologous chromosome.

Human female eggs remain in Meiosis I until puberty… 12-13 years

Meiosis I: Meiosis I: Metaphase I Metaphase I Each homologous set of

chromosomes line up along cell center

Orientation on the metaphase plate is random…with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Metaphase IMetaphase I

During metaphase I, the orientation of the homologous chromosome pairs is a matter of chance◦ each possible orientation of

which homologue in a homologous pair faces which pole results in gametes with different combinations of parental chromosomes

◦ this process is called independent assortment

Meiosis I: Meiosis I: Anaphase IAnaphase I

Homologs are pulled apart to opposite poles

Chromosomes, each with two chromatids, move to separate poles.

Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Meiosis I: Meiosis I: Telophase ITelophase I

Homologous chromosomes have fully separated

Results in a Haploid (1N) set of chromosomes at each pole

Nuclear envelopes may reform, or the cell may quickly start meiosis 2.

Overview: Meiosis IOverview: Meiosis I

Meiois IIMeiois II

Meiosis II is also divided into four stages

1.Prophase IInew spindle forms to attach to chromosome clusters

2.Metaphase IIspindle fibers bind to both sides of the centromere and individual chromosomes align along a central plane

3.Anaphase IIsister chromatids move to opposite poles

4.Telophase IIthe nuclear envelope is reformed around each of the four sets of daughter chromosomes

Meiosis IIMeiosis IIWhat does Meiosis II look like????

Meiosis OverviewMeiosis Overview

Meiosis vs. MitosisMeiosis vs. Mitosis

Why Sex??Why Sex?? Sexual reproduction has an

enormous impact on how species evolve because it rapidly generates new genetic combinations

Three mechanisms help produce this variety

1. Independent assortment2. Crossing over3. Random fertilization

GametogenesisGametogenesisGametogenesis the process of forming

gametes (haploid, 1n) from diploid cells (somatic cells) of the germ line.

Spermatogenesis is the process of forming sperm cells by meiosis. In spermatogenesis all 4 meiotic products develop into gametes and human males produce 200,000,000 sperm per day

Oogenesis is the process of forming an ovum (egg) by meiosis in specialized gonads known as ovaries. Human females female produce one egg (usually) each menstrual cycle

Gamete FormationGamete Formation