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
n
(23) = 23 chromosomes
Meiosis I
(each with 2 sister
chromatids)
Meiosis II
n
(23) = 23 single
chromosomes