Meiosis6.1, 6.2, 6.6

6.1 – Chromosomes & Meiosis

• Key Concept:• Gametes have half the number of chromosomes

that body cells have.

You have somatic cells and gametes.

• Somatic Cells:• Are body cells• Make up all cells in body except for

egg and sperm cells• DNA not passed on to children

• Gametes:• Are egg or sperm cells• DNA passed on

to children

Your cells have autosomes and sex chromosomes.

• Human somatic cells have 23 pairs of chromosomes (46 total)• (1) Autosomes: pairs 1 – 22; carry

genes not related to the sex of an organism

• (2) Homologous chromosomes: pair of • chromosomes; one from each parent; carry the

same genes but may have a different form of the gene (example: one gene for brown eyes and one gene for blue eyes)

• (3) Sex chromosomes: pair 23; determines the sex of an animal; control the development of sexual characteristics (Females XX, Males XY)

Stop & Review• Another name for body cells

• Somatic cells

• Another name for sperm & egg cells• Gametes

• The number of pairs of chromosomes in somatic cells• 23

• The number of individual chromosomes in somatic cells• 46

• Chromosome pairs 1-22• Autosomes

• Chromosome pair 23• Sex chromosomes

Somatic cells are diploid; gametes are haploid.

• Diploid (2n)• Has two copies of

each chromosome (1 from mother & 1 from father)• 44 autosomes, 2

sex chromosomes• Somatic cells are

diploid• Produced by mitosis

• Haploid (1n)• Has one copy of

each chromosome• 22 autosomes, 1

sex chromosome• Gametes are

haploid• Produced by

meiosis

• Chromosome number must be maintained in animals.

• Many plants have more than two copies of each chromosome (can be tetraploid [4n] or hexaploid [6n])

• Mitosis and meiosis are types of nuclear division that make different types of cells.

• Mitosis makes identical diploid cells.

• Meiosis makes different haploid cells from diploid cells.

Stop & Review• The number of copies of each chromosome in a diploid

cell• 2

• The number of chromosomes in a human diploid cell• 46

• The number of copies of each chromosome in a haploid cell• 1

• The number of chromosomes in a human haploid cell• 23

• This process produces genetically identical diploid cells• Mitosis

• This process produces genetically unique haploid cells• Meiosis

6.2 – Process of meiosis

• Key Concept:• During meiosis, diploid cells undergo two cell

divisions that result in haploid cells.

Cells go through two rounds of division in meiosis.

• Meiosis reduces chromosome number and creates genetic diversity.

• Homologous chromosomes (sometimes called homologues)• Pair of chromosomes• Inherit one from each parent• Carry same genes but code for different traits

(different versions of the gene)• Separate during Meiosis I

• Sister chromatids• Duplicates of each other• Each half of a duplicated chromosome• Attached together at the centromere• Separate in Meiosis II

Stop & Review

• These separate during Meiosis I• Homologous chromosomes

• These separate during Meiosis II• Sister chromatids

Meiosis I• Occurs after DNA has been replicated (copied)

• Divides homologous chromosomes in four phases.

Meiosis I

• (1) Prophase I• Chromosomes condense• Homologous chromosomes pair up• Nuclear envelope (membrane)

breaks down• Spindle fibers form• Crossing over occurs

Meiosis I

• (2) Metaphase I• Homologous chromosomes are lined up along

the middle of the cell (along the cell equator) by spindle fibers

Meiosis I

• (3) Anaphase I• Homologous chromosomes move apart to

opposite sides of the cell• Sister chromatids remain attached

Meiosis I

• (4) Telophase I & Cytokinesis• Spindle fibers fall apart• Nuclear membranes reform• Cytoplasm splits

Meiosis II

• Divides sister chromatids in four phases.

• DNA is not replicated between Meiosis I and Meiosis II.

Meiosis II

• (5) Prophase II• Nuclear envelope (membrane) breaks down• Spindle fibers form

Meiosis II

• (6) Metaphase II• Spindle fibers line chromosomes up along the

middle of the cell

Meiosis II

• (7) Anaphase II• Sister chromatids are pulled apart to opposite

sides of the cell

Meiosis II

• (8) Telophase II & Cytokinesis• Nuclear membranes form around chromosomes• Chromosomes begin to uncoil• Spindle fibers fall apart• Cytoplasm splits

Mitosis Vs. Meiosis

Mitosis

• One cell division

• Homologous chromosomes do not pair up

• Results in 2 diploid cells

• Daughter cells are identical to parent cell

Meiosis

• Two cell divisions

• Homologous chromosomes pair up (Metaphase I)

• Results in 4 haploid cells

• Daughter cells are unique

Stop & Review – Mitosis Or Meiosis?

• This involves two rounds of cell divisions• Meiosis

• This produces 4 genetically unique haploid daughter cells• Meiosis

• This results in daughter cells that are identical to the parent cell• Mitosis

• Homologous chromosomes pair up during this process• Meiosis

• This results in 2 genetically identical daughter cells• Mitosis

• This involves one round of cell division• Mitosis

Haploid cells develop into mature gametes.

• Gametogenesis is the production of gametes.

• Gametogenesis differs between males and females.• Sperm (spermatogenesis)

• Become streamlined and motile (able to move)• Primary contribution to embryo is DNA only• Males produce over 250 million sperm per day

• Egg (oogenesis)• Contribute DNA, cytoplasm, and organelles to the

embryo• During meiosis, the egg gets most of the contents, the

other 3 cells become polar bodies• Females produce a few hundred eggs over a lifetime

6.6 – Meiosis & Genetic Variation

• Key Concept:• Independent assortment and crossing over

during meiosis result in genetic diversity.

Sexual reproduction creates unique combinations of genes.• Fertilization

• Random• Increases unique combinations of genes• In humans, the chance of getting any one

combination of chromosomes from any one set of parents is one out of 223 x 223 (which is one out of over 70 trillion combinations)

Sexual reproduction creates unique combinations of genes.

• Independent assortment of chromosomes• Homologous chromosomes line up randomly

along the cell equator• Increases the number of unique combinations of

genes• In human cells, about 223 (8 million) different

combinations could result

Sexual reproduction creates unique combinations of genes.

• Crossing over• Exchange of chromosome segments between homologous

chromosomes• Increases genetic diversity• Occurs during Prophase I of Meiosis I• Results in new combinations of genes (chromosomes have

a combination of genes from each parent) http://www.youtube.com/watch?v=5x_Rp1mwotQ

Genetic linkage

• Chromosomes contain many genes.• The farther apart two genes are located on a

chromosome, the more likely they are to be separated by crossing over

• Genetic linkage: genes located close to each other on the same chromosome tend to be inherited together