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Meiosis

How are human and animal chromosomes categorized?

• Sex chromosomes- the 23rd pair of chromosomes determines the gender of an organism; and they also carry genes for other traits.

• Female = XX• Male = XY

• Autosomes- All other chromosome pairs (1-22) which code for all the other traits.

How many chromosomes are there in human cells?

• Humans have 23 pairs or a total of 46 chromosomes.

What are homologous chromosomes?Every organism produced by sexual reproduction

has two copies of each autosome(one from each parent)

Homologous chromosomes or homologues : Two copies of each autosome

- Same size - Same shape- nCarry genes for the same trait

Journal:

What is the difference between sister chromatids and homologues?

Diploid number of chromosomes

• Cells having two sets of chromosomes are said to be diploid; they have both chromosomes for each homologous pair.– All body cells have a diploid number of

chromosomes

• Also referred to as 2n, 2(23)=46

Haploid number of chromosomes• Cells contain only one set of chromosomes;

they have half the normal or diploid number of chromosomes.

• Only the reproductive cells called gametes (sperm and egg cells in humans) have haploid number.

• Also referred to as 1n, 1(23)=23

Haploid (n=7)

Diploid (2n = 14)

Haploid Gametes 1n (Reproductive Cells)

Diploid zygote 2n (Offspring- also called a zygote)

Journal:

• What is the difference between haploid and diploid?

Meiosis

• Nuclear division that reduces the number of chromosomes in the new cells produced half_(haploid) the number in the original cell.– In males this takes place in the sperm. It is

called spermatogenesis.– In females this takes place in the egg and it

is called oogenesis.

Meiosis has 2 parts

• Meiosis I- Homologous chromosomes are separated

• Meiosis II- _Sister chromatids are separated

• Meiosis I– Prophase I- chromosomes are paired with

homologues to form tetrads– M etaphase I- Homologues meet in the middle– A naphase I- Homologues are pulled apart– T elophase I- Two nuclei are formed– Cytokinesis- Splits into two haploid cells

• Meiosis II– Prophase II- Nuclear membrane disappears– M etaphase II- Chromosomes meet in the middle– A naphase II- Sister Chromatids are pulled apart – Telophase II- Two nuclei form– Cytokinesis- Four haploid cells are formed

Meiosis Provides Genetic Variation• Recombination: Prophase I homologues

chromsomes _cross over and genetic material may be exchanged.

Meiosis Provides Genetic Variation• Law of Independent Assortment: Homologues line up independently of each other!

Journal

What are the 2 ways that Meiosis provides for genetic variation?

Nondisjunction

• When homologous chromosomes _fail_ to separate properly during meiosis. – Results• A gamete with an extra chromosome (trisomy)• A gamete missing a chromosome (monosomy)

Trisomy 21

Polyploidy• Organisms with more than the usual number

of chromsomes– Occurs in plants– Caused by self fertilization

THE END!