#13330 - Meiosis - Organisation of the Body

Meiosis

Key terms

PLOIDY- refers to the number of copies of each chromosome present in the cell nucleus

N (number)- refers to the number of copies of each unique double stranded DNA molecule in nucleus

-haploid gametes with one DNA molecule per chromosome are 1N

-diploid cells with one DNA per chromosome (mother and father)- 2N

Meiosis- process of cell division that occurs only in the germ line. Diploid germ cell undergoes 2 successive nuclear and cell division to form 4 haploid gametes that are 1N cells

Stages

-preliminary step in meiosis is the replication of chromosomal DNA molecule, diploid cell is converted from 2N to 4N. Once the DNA replicates, there are 2 chromatids are joined together at a structure called the centromere

This stage forms the primary oocyte and in males the primary spermatocyte

Meiosis 1- reduction cell division

Prophase

-chromosomes condense into a compact double stranded structure

-late stages of prophase, the double stranded chromosomes of each homologous pair match up, centromere to centromere, to form a joint structure known as a chiasma

-chiasma formation makes is possible for 2 homologous chromosomes to exchange large segments of DNA by a process called crossing over- exchange of maternal and paternal genes

-recombination of genetic material on homologous maternal and paternal chromosomes is random and increases the genetic variability of the future gametes

At this stage, the primary oocyte enters a phase of meiotic arrest

Metaphase

-four stranded chiasma structures are organised on the equator of spindle apparatus

-the homologous pair of chromosomes align randomly at the metaphase plate- independent segregation- they separate randomly- different combination of maternal and paternal chromosomes in the daughter cells

Anaphase

-one double stranded chromosome of each homologous pair is distributed to each of the 2 daughter nuclei

-the resulting daughter cells are haploid but are 2N as two chromatids of each chromosome remain together

-they contain the same amount of DNA as the parent cell but half the number of chromosomes

-the daughter nuclei form and the cell undergoes cytokinesis

This forms two secondary spermatocytes in the male and a secondary oocyte and a first polar body in the female

Meiotic Division 2

-no DNA replication occurs- no preceding S phase unlike Meiotic division 1

Prophase

• The 23 double stranded chromosomes condense during the second meiotic phase

Metaphase

• The chromosomes line up at the equator

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