Download Meiosis PPT

Mitosis vs. Meiosis
Meiosis
• The form of cell division by which gametes, with half
the number of chromosomes, are produced.
• Diploid (2n)
→
haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Meiosis
• Sex cells divide to produce gametes (sperm or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some chromosomal
differences.
Spermatogenesis
human
sex cell
meiosis I
sperm
meiosis II
Meiosis I (four phases)
• Cell division that reduces the chromosome number
by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are similar in
shape and size.
• Homologous pairs (tetrads) carry genes controlling the same
inherited traits.
• Each locus (position of a gene) is in the same position on
homologues.
• Humans have 23 pairs of homologous chromosomes.
a.
b.
22 pairs of autosomes (somatic cells)
01 pair of sex chromosomes
• Gene- unit of heredity that is transferred
from parent to offspring and is held to
determine characteristics of the offspring
• Trait- feature or characteristic of a living
thing
• Locus- specific location of a gene, or DNA sequence, on
a chromosome
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Karyotype
• A method of organizing the chromosomes of a cell in relation to
number, size, and type.
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the
“Sex
Chromosomes”
are the 23rd set
XX chromosome - female
XY chromosome - male
Boy or Girl? The Y Chromosome
“Decides”
Y chromosome
X chromosome
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister chromatids break
and reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Another Way Meiosis Makes Lots of Different Sex
Cells – Crossing-Over
Crossing-over multiplies the already huge number of different
gamete types produced by independent assortment.
Question:
• In terms of Independent Assortment how many different combinations of
sperm could a human male produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
Anaphase I
• Homologous chromosomes separate and move
towards the poles.
• Sister chromatids remain attached at their
centromeres.
Non Disjunction
• Occurs when chromosomes fail to separate properly
– Some cells end up with one more or one less pair of
chromosomes
21 trisomy – Downs Syndrome
Can you see the
extra 21st
chromosome?
Is this person male
or female?
Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Variation
• Important to population as the raw material for
natural selection.
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes