Transcript Meiosis Powerpoin

CHROMOSOMES
•DNA (deoxyribonucleic acid) is a long, thin molecule that
directs cellular functions and heredity.
•DNA contains information that is encoded in segments called
genes.
•A single molecule of DNA contains thousands of genes.
Chromosome Structure
•A chromosome forms when a
single DNA molecule, and its
associated proteins (histones),
coil tightly prior to cell division.
•DNA only coils in the form of
chromosomes when the cell is
preparing for cell division
By the time chromosomes coil and
become visible, the DNA has been
copied, and each chromosome
consists of two copies ensuring
that each new cell will have the
same genetic information.
•The identical strands of DNA are
each called chromatids and are
held together by a centromere.
•Body cells (somatic cells) contain two copies of each
chromosome, referred to as homologous chromosomes.
•Homologous chromosomes are similar in size, shape and have the
same type of genetic information (code for the same
characteristics).
•One homologue originates
from the female parent
and the other from the
male parent.
•Human cells have 23
different PAIRS of
homologous chromosomes.
–1 pair is the sex
chromosomes(determine
sex and related characteristics)
•In humans and many other species, females are
designated XX and males XY. are designated
–22 pairs are autosomes (determine other traits)
•The number of chromosomes in
cells is constant within a species.
•Diploid cells (2n)contain two
homologues of each chromosome.
–Somatic cells
•Haploid cells (n)contain one
homologue of each chromosome.
–Sex cells –gametes(egg and
sperm)
•For humans 2n = 46. (∴n = ?)
For fruit flies 2n = 8. (n = 4)
The haploidg ametes are fused in fertilization which restores
the diploid number in the zygote(the fertilized egg).
A karyotype is a pictoral
spread of an individual’s
chromosomes.
•It can be used to detect
some genetic
abnormalities (Down’s
Syndrome)
MEIOSIS
•Meiosis is a form of cell division that halves the number of
chromosomes in cells.
•Meiosis takes place in order to make gametes
•Four haploid gametes are produced from one diploid cell.
•Meiosis consists of two successive nuclear divisions.
Interphase
• Replication
of DNA
must occur
before cell
division
Prophase I
• Chromosomes
condense
• Nuclear
membrane
disappears
• Spindle fibers
begin to form
• Homologous
chromosomes pair
Prophase I cont.
• Homologous
chromosomes
pair to form a
tetrad
• Crossing over
results in
genetic
variation
CROSSING OVER
•Crossing over(exchange of genetic material between
homologues) occurs during Prophase I. It produces genetic
recombination.
Metaphase I
• Homologous
chromosomes
line up along
the equator of
the cell
Anaphase I
• Homologous
chromosomes
separate
move to
opposite
poles
Telophase I
• Two cells are
formed
• Each has ½ the
genetic
information of
the parent cell
Overview of Meiosis I
Prophase II
• Condensed
chromosomes
attach to
spindle fibers
• Nuclear
membrane
disappears
Metaphase II
• Chromosomes
line up along
equator of
cell
Anaphase II
•Sister
chromatids
are pulled
apart
Telophase II
•Each cell
forms 2
new (1n)
cells
Meiosis II
• Meiosis II is much like
mitosis with chromatids
separating
• End result of meiosis is 4
cells with half the genetic
material as the parent cell
Overview of Meiosis II
Comparing Meiosis & Mitosis
Mitosis
Meiosis
•Homologous
chromosomes
independent
•Homologous
chromosomes pair,
forming homologues until
anaphase I
•Identical daughter cells
•Daughter cells haploid
Daughter cells have new
assortment of parental
chromosomes
•Chromatids not identical;
crossing over
Formation of Gametes
In animals, meiosis produces haploid reproductive cells called gametes
Only occurs within their reproductive organs
In humans, meiosis occurs in the testes and in the ovaries
Testes – production of male gametes known as sperm cells. Diploid
reproductive cell divides meiotically to form four haploid cells called
spermatids.
Production of sperm cells is called spermatogenesis
This process produces 4 viable sperm cells
Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes
http://www.djusd.k12.ca.us/davishigh/amoriarty/Meiosis.htm
Formation of Gametes
Ovaries- Oogenesis is production of mature egg cells
Diploid reproductive cell divides meiotically to produce one
mature egg cell
During cytokinesis I and II of oogenesis, cytoplasm of the
original cell is divided unequally between new cells
One egg cell is produced by meiosis.
Other three products of meiosis called polar bodies,
degenerate (break down)
Mistakes in Meiosis
• Mistakes in meiosis cause genetic
abnormalities
• One such genetic abnormality is Down
syndrome or Trisonomy 21
Meiosis – division error
Chromosome pair
Meiosis error - fertilization
Should the gamete with
the chromosome pair be
fertilised then the
offspring will not be
‘normal’.
In humans this often
occurs with the 21st pair
– producing a child with
Downs Syndrome
21 trisomy – Downs Syndrome
Can you see
the extra 21st
chromosome?
Is this person
male or
female?