Download Ch. 13 Meiosis - HobbsAPBiology

Ch. 13 Meiosis
I. Introduction
A. Chromosomes and their arrangement
1. In humans, Somatic cells (norm body)
have 46 chromosomes
Chromosomes differ in
Size
Centromere position
Stained banding
patterns
I. Introduction
A. Chromosomes and their arrangement
2. Homologous Pairs
Each chromosome has a partner
similar in appearance.
Carry genes for the same traits
Called Homologous pair
Somatic cells have 23 pairs
I. Introduction
A. Chromosomes and their arrangement
3. A Karyotype arranges chromosomes in
homologous pairs
I. Introduction
A. Chromosomes and their arrangement
4. The sex chromosome pair
One pair determines gender
XX = female
XY = male
The other 22 pairs are called
Autosomes
I. Introduction
A. Chromosomes and their arrangement
5. Chromosome arrangement in sexual
reproduction
One chromosome from each pair is
inherited from each parent.
In each of your homologous pairs
one chrom. Is maternal
the other is paternal
You have a set of 23 from each
I. Introduction
A. Chromosomes and their arrangement
5. Chromosome arrangement in sexual
reproduction
Gametes (sex cells) have 23, one out of
each homologous pair.
22 autosomes, and a X or Y
23 = Haploid (n)
I. Introduction
A. Chromosomes and their arrangement
5. Chromosome arrangement in sexual
reproduction
The Zygote (fertilized egg) gets two
haploid sets
a maternal set from egg
a paternal set from sperm
It is now Diploid (2n) = 46
I. Introduction
B. The Role of Mitosis
To replicate diploid somatic cells from
other diploid somatic cells
I. Introduction
C. The Role of Meiosis
To make Gametes, haploid (n) cells from
diploid cells
Chromosome number is halved to make
sperm and egg
II. Meiosis and Life Cycles
A.Meiosis and
fertilization alternate
Meiosis halves the
genome
Fertilization doubles
the genome
B. In humans,
gametes are the
only haploid stage
II. Meiosis and Life Cycles
C. In other organisms, the haploid stage is
more prominent
1. Most fungi and some protists
Zygote (2n)
Meiosis
Multicellular adult (n)
Mitosis
Gametes (n)
Fertilization - Zygote
II. Meiosis and Life Cycles
2. Plants - Alternation of Generations
Sporophyte (2n)
Meiosis
Spores (n)
Mitosis
Gametophyte (n)
Mitosis
Gametes (n)
Fertilization
Zygote (2n) ….. Sporophyte (2n)
III. The Process of Meiosis
A. Chromosomes replicate first as in
mitosis
Each chromo and its copy =
Sister Chromatids
B. Followed by TWO Divisions
Meiosis I
Meiosis II
C. Results in 4 daughter cells (n)
D. Cuts chromosome number in half
Replicates once, but divides twice.
III. The Process of Meiosis
E. Meiosis I - Separates homologous pairs
1. Prophase I - Homologous
chromosomes pair up - forms tetrads
III. The Process of Meiosis
E. Meiosis I - Separates homologous pairs
1. Prophase I - Within tetrads,
Crossing Over may take place
Chromosomes cross and trade sections
Spindle forms as usual
III. The Process of Meiosis
E. Meiosis I - Separates homologous pairs
2. Metaphase I - Tetrads center in spindle
as usual
III. The Process of Meiosis
E. Meiosis I - Separates homologous pairs
3. Anaphase I - Homologous pairs are
separated
Sister chromatids remain attached
III. The Process of Meiosis
E. Meiosis I - Separates homologous pairs
4. Telophase I and Cytokinesis as in normal
mitosis
III. The Process of Meiosis
F. Meiosis II - Similar to Mitosis
1. Prophase II - normal spindle action
2. Metaphse II
Sister chromatids line up on the
metaphase plate
3. Anaphase II - sister chromatids are
separated
4. Telophase and Cytokinesis - normal
III. The Process of Meiosis
G. Meiosis ends with
4 haploid daughter cells
H. Key differences between Mitosis and
Meiosis
1. Resultant chromosome number
2. Meiosis cells are not clones
Mitosis cells are
3. Number of divisions
III. The Process of Meiosis
H. Key differences between Mitosis and
Meiosis
events are unique to Meiosis I
4. In Prophase I, Homologs held
together by Synapsis
Protein zipper - synaptonemal
complex
5. Chromosomes together as Tetrad
6. Crossing over - sections exchanges
at junctions called Chiasmata
III. The Process of Meiosis
H. Key differences between Mitosis and
Meiosis
7. Separations differ
8. Meiosis makes 4 daughter cells.
IV. Origins of Genetic Variation
A. The Production of Genetic variation
1. Three mechanisms (Outline)
Independent Assortment
Crossing Over
Random Fertilization
IV. Origins of Genetic Variation
2. Independent Assortment
The random orientation of tetrads along
the metaphase plate.
Each one orients randomly, independent of
how the other tetrads arrange.
From each homologous pair, a gamete
could get the paternal one or maternal
one
Given 23 pairs, there are many possible
combinations of maternal and paternals
IV. Origins of Genetic Variation
3. Crossing Over - produces
Recombinant chromosomes
Crossing over breaks linked genes
(genes on the same chromosome)
This creates new combinations of traits
IV. Origins of Genetic Variation
4. Random Fertilization - Any sperm can
fertilize any egg.
Each sperm and egg is unique
Each one has unique combinations
of paternal and maternal
chromosomes.
Each one has unique
recombinations due to crossing
over.
A zygote can result from any sperm with
any egg
IV. Origins of Genetic Variation
B. Importance of Genetic Variation
The raw material for evolution.
Genetic variation provides the range of
traits for natural selection.
Less genetic variability =
lower ability to adapt and change