Chromosome Number: A refresher

1
Chromosome NumberA refresher

• Homologous- term used to refer to chromosomes
  that each have a corresponding chromosome from
  the opposite-sex parent
• Somatic cells (regular body cells)- have 23 pairs
  of homologous chromosomes or 46

2
Not all organisms have the same number of
chromosomes
3
Figure 13.x3 Human female karyotype shown by
bright field G-banding of chromosomes
23 pairs of homologous chromosomes
4
Chromosome Number
Diploid

• Diploid (2N) - term used to refer to a cell that
  contains both sets of homologous chromosomes
• Haploid (N) - term used to refer to a cell that
  contains only a single set of chromosomes and
  therefore only a single set of genes

Haploid
5

• Somatic cells- regular body cells (diploid)
• Germ cells- cells used for sexual recombination
  (haploid)

6
Why do gametes have only a single set or
chromosomes?
7
(No Transcript)
8
Genetic Recombination and its Importance

• Genetic recombination takes place when organisms
  sexually reproduce.
• For reproduction organisms produce specialized
  cells that only have a single set of chromosomes
  (Gametes)
• The sperm and egg are gametes
• Gametes are haploid

Genetic recombination causes diversity
9
Fruit Fly Chromosomes These chromosomes are from
a fruit fly (Drosophila). Each of the fruit flys
body cells has 8 chromosomes. There are 4
homologous chromosomes- Four of the chromosomes
came from the fruit flys male parent, and 4 came
from its female parent.
10
Meiosis

• Type of cell division where germ cells produce
  gametes (sperm and eggs), each containing half
  the number of chromosomes (haploid number) as a
  parents body cell

11
Goal of Meiosis

• To reduce the number of chromosomes from a
  diploid number to a haploid number. The new
  cells that result are called gametes, and can
  join together with their counterparts (egg with
  sperm, sperm with egg) to produce a new diploid
  organism.
• Meiosis is also called reduction division

12
Figure 13.7 The stages of meiotic cell division
Meiosis I
13
Figure 13.7 The stages of meiotic cell division
Meiosis II
14
Figure 13.6 Overview of meiosis how meiosis
reduces chromosome number
15
Crossing Over

• As homologous chromosomes pair up and form
  tetrads in meiosis I, they may exchange portions
  of their chromatids in this process.results in
  an exchange of genetic material and gives new
  gene combinations.

16
Figure 13.10 The results of crossing over during
meiosis
17
While in a tetrad crossing over takes place
Crossing-Over
Section 11-4
Go to Section
18
Figure 13.8 A comparison of mitosis and meiosis
summary
19
Comparing Meiosis to Mitosis

• Mitosis results in the production of two
  genetically identical diploid cells, whereas
  meiosis produces four genetically different
  haploid cells