Cell Division-Meiosis

How it leads to Variation

A Cells Cycle1. G1 – growth

2. S = Synthesis (to make) – Copy the DNA

3. G2 – growth

4. M – Cell Division (mitosis or meiosis)

5. G0 is a cell that doesn’t divide

Life Cycle of Humans

Binary Fission – one cell divides to become 2 - Bacteria Only

Mitosis – All other life - Identical copies of body cells are made

Meiosis – All other life - Gametes (sex cells) unique cells with half of the original DNA are made

Types of Cell Division

Meiosis - General1. Starts with one diploid (2 sets of chromosomes) cell

2. 2 cell divisions

3. Ends with 4 different cells that are haploid (1 set of DNA)

Process of Meiosis1. Before Cell Division a cell MUST copy its DNA.

2. In Meiosis I (first division) – The homologous chromosomes made up of sister chromatids go through crossing over.

Crossing Over – When homologous chromosomes exchange pieces causing variation in the final cells.

Steps of Crossing Over1. Homologous chromosomes pair up

2. Homologous chromosomes intertwine and swap similar portions of DNA

3. New recombined chromatids on the homologous chromosomes are formed.

Process of Meiosis3. In Meiosis I –

Independent assortment – When each pair (23 pairs) of homologous chromosomes line up independently of each other causing variation in the final cells.

Process of Meiosis4. In Meiosis II (second division) – Starts with 2 haploid cells.

The sister chromatids in each of these 2 cells are separated into the final four haploid cells .

Mistakes = VariationNondisjunction – When chromosomes fail to

separate correctly during meiosis. This leads to gametes with extra chromosomes.

KaryotypeA karyotype is an organized profile of an individual’s

chromosomes. In a karyotype, chromosomes are arranged and numbered by size, from largest to smallest.

Humans have 46 chromosomes or 23 pairs.

22 of these pairs are autosomes.

1 pair are sex chromosomes, X and Y.

An XX person = female; an XY individual = male

NondisjunctionIf one of these sex cells fuses with another to form a zygote,

that person could have an extra or missing chromosome.