Download Ch. 9 Meiosis

Meiosis
Ch. 9
Gametes

Gametes are reproductive cells (eggs
and sperm) that contain half the
complement of chromosomes found in
somatic cells
◦ the gametes fuse to form a new cell called
a zygote, which contains two complete
copies of each chromosome
 the fusion of gametes is called fertilization, or
syngamy
egg + sperm = Zygote
Haploid Cell vs. Diploid Cell

Typically, each cell of your
body has 46
chromosomes..23 from each
parent

So, you have what we call a
Diploid value of 46

Or, referred to as 2N = 46

Your gametes, however, have
1N values

1N = 23….This is a Haploid
condition

All your normal body cells
are diploid, only your
gametes are haploid
◦ THIS IS TRUE FOR MOST
ORGANISMS!!!!!!
Sex Cells
Chromosome Number
•
Every organisms inherits chromosomes from their
parents.
• Asexually reproducing organisms pass on their
full set of chromosome creating offspring that are
essentially clones of themselves
• Sexually reproducing organisms pass on half of
their chromosomes through their sex cells. The
offspring created inherits half of its chromosomes
from each parent.
Human Life Cycle
Gamete- egg and
sperm
 Zygote- cell
produced when egg
and sperm fuse
 Fertilization/
syngamy- fusion of
gametes

Traits
•
Inheritable traits are carried on factors called genes and are passed
down from generation to generation.
•
Genes are located on DNA within the nucleus of the cell.
•
There are two alleles for every gene. One allele is inherited from
each parent
• Alleles are alternate versions of a gene
•
The process by which chromosomes are separated in the production
of sex cells is known as Meiosis.
Homologous Chromosomes
•
Homologous
Chromosomeschromosomes that carry
the alleles for the same
gene.
• Diploid Cells- (2N) “Two
sets”; have both sets of
homologous
chromosomes (All body
cells are diploid)
• Haploid Cells- (1N) “one
set”; has a single set of
genes. (All sex cells are
haploid)
Meiosis
Meiosis- is the process by which
the homologous chromosomes
are separated in a diploid cell to
produce a haploid cell
Meiosis involves two divisions,
meiosis I and meiosis II
◦ meiosis I separates the
homologues in a homologous
pair
◦ meiosis II separates the
replicate sister chromatids
Meiosis I

Meiosis I is traditionally divided into four sequential
stages
1.
Prophase I

2.
Metaphase I

3.
The paired homologous chromosomes align on a central plane
Anaphase I

4.
Homologous chromsomes pair up and exchange segments
Homologues are pulled apart and move to opposite poles of
the cell
Telophase I

Individual replicated chromosomes gather at each of the two
poles
Prophase I




Prior to the start of Meiosis,
chromosomes are replicated
during Interphase.
During Prophase I homologous
chromosomes pair up to form a
tetrad (there are 4 chromatids in a
tetrad)
Crossing over may occur when the
tetrads exchange a portion of the
chromatids
◦ During crossing-over chromatids
break and may be reattached to
a different homologous
chromosome.
Human female eggs remain in
Meiosis I until puberty… 12-13
years
Meiosis I: Metaphase I


Each homologous set of
chromosomes line up along cell
center
Orientation on the metaphase plate
is random…with either parental
homologue on a side. This means
that there is a 50-50 chance for the
daughter cells to get either the
mother's or father's homologue for
each chromosome.
Metaphase I

During metaphase I, the
orientation of the homologous
chromosome pairs is a matter of
chance
◦ each possible orientation of
which homologue in a
homologous pair faces which
pole results in gametes with
different combinations of
parental chromosomes
◦ this process is called
independent assortment
Meiosis I: Anaphase I



Homologs are pulled apart
to opposite poles
Chromosomes, each with
two chromatids, move to
separate poles.
Each of the daughter cells
is now haploid (23
chromosomes), but each
chromosome has two
chromatids.
Meiosis I: Telophase I
Homologous chromosomes have fully
separated
 Results in a Haploid (1N) set of
chromosomes at each pole
 Nuclear envelopes may reform, or the
cell may quickly start meiosis 2.

Overview: Meiosis I
Meiois II
Meiosis II is also divided into four stages
1.Prophase II
new spindle forms to attach to chromosome clusters
2.Metaphase II
spindle fibers bind to both sides of the centromere and individual
chromosomes align along a central plane
3.Anaphase II
sister chromatids move to opposite poles
4.Telophase II
the nuclear envelope is reformed around each of the four sets of
daughter chromosomes
Meiosis II
What does Meiosis II look like????
Meiosis Overview
Meiosis vs. Mitosis
Why Sex??

Sexual reproduction has an enormous impact
on how species evolve because it rapidly
generates new genetic combinations

Three mechanisms help produce this variety
1.
2.
3.
Independent assortment
Crossing over
Random fertilization
Gametogenesis



Gametogenesis the process of forming gametes
(haploid, 1n) from diploid cells (somatic cells) of the
germ line.
Spermatogenesis is the process of forming sperm
cells by meiosis. In spermatogenesis all 4 meiotic
products develop into gametes and human males
produce 200,000,000 sperm per day
Oogenesis is the process of forming an ovum
(egg) by meiosis in specialized gonads known as
ovaries. Human females female produce one egg
(usually) each menstrual cycle
Gamete Formation