Chromosomes and Meiosis

Recall,

• DNA on each chromatid is

Sectioned into many segments called

• Genes: govern the expression of a particular trait

Diversity of chromosomes among organisms

• The number and length of chromosomes differ among organisms

• There is no relationship between size of an organism and number of chromosomes

Numbers of Pairs of Chromosomes in Some Plant and Animal Species

Common name Species # chrom. Pairs

Mosquito Culex pipiens 3

Housefly Musca domestica 6

Toad Bufo americanus 11

Rice Oryza sativa 12

Frog Rana pipiens 13

Alligator Alligator mississippienis 16

Rhesus monkey Macaca mulatta 21

Wheat Triticum aestivum 21

Human Homo sapiens 23

Potato Solanum tuberosum 24

Donkey Equus asinus 31

Horse Equus caballus 32

Dog Canis famliaris 39

Carp Cyprinus carpio 52

Most animal species contain two types of cells:

• 1. Somatic/Body Cells– Contain two sets of chromosomes written as

2n (called diploid cells)

• 2. Gamete/Sex Cells– Contain one set of chromosomes written as n

(called haploid cells)

• (I.e. Humans)

– Somatic Cells = 2n (46 chromosomes)– Sperm/egg Cells = n (23 Chromosomes)

Humans have 46 chromosomes or 23 pairs of chromosomes [ 2(23) ]

• 22 pairs = autosome chromosomes– not involved in sex

determination

• 1 pair = sex chromosomes– responsible for sex

determination

Why do we need meiosis?

• If all cells including sex cells/gametes were diploid (2n) then….

2n (male) 2n (female)

4n (offspring) ?

Meiosis:

• Occurs in reproductive organs

• Produces gametes (haploid cells) to maintain the diploid number in offspring

• Involves 2 sequences of phases– each similar to mitosis– Called meiosis I and meiosis II

Meiosis I:

• DNA replication

– (Same as mitosis)

• Homologous chromosomes – Identical in size and

appearance– Corresponding genes may

differ in form

• One chromosome comes from the female gamete and one from the male gamete

• Homologous chromosomes pair to form homologous pairs– All 4 chromatids are

called a Tetrad

• One spindle from one pole attaches to one chromosome

• One spindle from the other pole attaches to the other

• Tetrads are pulled to the equator of the cell– Not in single file

• Homologous chromosomes separate and move to opposite poles of the cell– Centromere does not

split – holds sister chromatids together

• The chromosomes gather into nuclei, and the original cell divides

• Brief interphase:– No DNA replication

Results of Meiosis I:

• Reduction division– Chromosome number is reduced by half

• 2n n

• Each daugher cell going into Meiosis II is haploid (n)

Meiosis II

• (same as mitosis)

(see notes from last class)

• Same as mitosis

• Same as mitosis

• Same as mitosis

RESULT FROM MEIOSIS II:

• 4 Haploid daughter cells

• Containing single unreplicated chromosomes

• Gamete cells (n)

P.162

Recap:

2n 2(23) = 46 chromosomes

Meiosis I n (23) = 23

chromosomes (each with 2 sister

chromatids)

Meiosis II n (23) = 23 single

chromosomes