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Meiosis and Sexual Life Cycles
Sexual Reproduction
Reproduction = process by which a new
generation of cells or multicellular individuals is
produced.
Sexual reproduction requires meiosis, formation of
haploid gametes (eggs and sperm), and
fertilization (union of a haploid egg and a haploid
sperm to form a diploid zygote).
The Role of Meiosis in Sexual Life Cycles
• Ploidy refers to the number of chromosome sets in a cell.
– Diploid (2n) - condition in which cells contain two sets of
chromosomes (one set from each parent)
– Haploid (n) - cells contain one set of chromosomes. In
animals, these cells are called gametes (sperm and egg cells),
in plants – pollen and egg.
• Meiosis is the process by which the chromosome number is
halved during gamete formation. Thus, meiosis reduces
chromosome number from diploid to haploid.
• The human life cycle -- follows the same basic pattern found in
all sexually-reproducing organisms; meiosis and fertilization
result in alternation between haploid and diploid condition.
Review of Terminology:
• Autosome -- any chromosome except sex chromosomes.
• Homologue -- a pair of chromosomes that have the same size,
centromere position, and banding pattern
– With one exception (sex chromosomes in a male), homologues
carry the same genetic loci.
– However, the sequence of the two genes may differ at a
particular locus on the two chromosomes. Different forms of a
gene for the same trait are called alleles.
• Sex Chromosomes -- dissimilar chromosomes that determine an
individual’s sex.
– Human females have a pair of X chromosomes.
– Human males have one X and one Y chromosome.
– Thus humans have 22 pairs of autosomes and one pair of sex
chromosomes.
maternal
homologue
one
homologous
pair of
chromosomes
paternal
homologue
Activity Biozone:Eukaryote chromosome structure
46 chromosomes from a human male.
Each chromosome is in the duplicated state.
They are arranged as pairs of homologous chromosomes, with
chromosome 1 the largest and chromosome 22 the smallest.
Note the small size of the Y chromosome as compared to
the X chromosome.
1
2
3
9
10
11
4
5
12
13
6
14
7
8
15
16
Y
17
18
19
20
21
22
X
Skin Color
Height
Homologous chromosomes mean that diploid cells have
two copies of a gene for each trait, with one copy coming
from mom and one copy coming from dad!
However, the sequences of the two genes do not have to be identical.
Alternative forms of genes for the
same trait at the same location on a chromosome are called alleles.
2n
Meiosis occurs as
two consecutive
cell divisions:
Meiosis I
(splitting pairs of
homologous
chromosomes)
and
Meiosis II
(splitting sister
chromatids apart).
n
Meiosis: Two Divisions
Meiosis I
Meiosis II
Key Differences Between Meiosis and Mitosis
Mitosis is characterized by just one division:
diploid state is retained and 2 daughter cells are
genetically identical to parental cell.
Meiosis is two successive nuclear divisions:
• Meiosis I is a reduction division (one diploid cell
divides to form two haploid cells).
• Meiosis II is just like mitosis, except that the cells are
haploid at the beginning of Meiosis II.
Overall, meiosis produces four haploid daughter cells
(which will become gametes), which are all genetically
different from one another. In males 4 sperm/pollen are
produced. In females one egg survives.
Nb Haploid = 1 allele set, Diploid = paired allele set
MEIOSIS I
plasma
membrane
newly
forming
microtubules
in the
cytoplasm
PROPHASE I
spindle
equator
(midway
between the
two poles)
one pair of
homologous
chromosomes
METAPHASE I
ANAPHASE I
TELOPHASE I
Meiosis I cuts chromosome number in half: cells change from
diploid to haploid state.
Note that each chromosome is still duplicated, though.
MEIOSIS II
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
Meiosis II is similar to mitosis.
Duplicated chromosomes line up randomly at metaphase, and
sister chromatids split apart to form two separate chromosomes.
See the following Web sites for a description and
animation of meiosis and a meiosis problem set!
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/problems.html
The stages of meiotic cell division.
• Interphase I
– Chromosomes replicate as for mitosis.
– Each duplicated chromosome consists of two
identical sister chromatids attached at their
centromeres.
– Centrosomes duplicate.
Prophase I
Chromosomes condense.
Homologous chromosomes come together as pairs
(called synapsis).
Crossing-over occurs between nonsister chromatids
of homologous chromosomes (see later).
Centrosomes migrate to opposite poles of cell and
spindle microtubules begin to form.
Nuclear envelope and nucleoli disperse.
Chromosomes begin to move to center of the cell.
Prophase I occupies >90% of the time required for
meiosis.
MEIOSIS - PROPHASE I
MEIOSIS - METAPHASE I
Pairs of homologous chromosomes are aligned in
center of cell.
Each pair is aligned so that centromeres of homologues point
toward opposite poles.
Each homologue is attached to kinetochore microtubles
emerging from the pole it faces.
MEIOSIS - ANAPHASE I & TELOPHASE I
Anaphase I
Homologous chromosomes separate and the
chromosomes are moved toward opposite
poles by the spindle apparatus.
Telophase I and Cytokinesis I
The spindle apparatus continues to separate
homologous chromosomes until the
chromosomes reach the opposite poles.
Cytokinesis occurs simultaneously with
telophase I, forming two haploid daughter
cells (but with each chromosome still in its
duplicated state).
Meiosis II-This second meiotic division separates sister
chromatids of each chromosome.
Prophase II
Spindle apparatus forms and chromosomes
move toward the center of the cell.
Metaphase II
Chromosomes align, randomly and singly, at
the center of the cell.
MEIOSIS - PROPHASE II & METAPHASE II
MEIOSIS - ANAPHASE II & TELOPHASE II
Anaphase II
Centromeres of sister chromatids separate and move towards opposite poles.
Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell.
Cytokinesis occurs producing four haploid daughter cells.
germ cell
2n
germ cell
each chromosome
duplicating during
interphase
MEIOSIS I
separation of
homologues
n
MEIOSIS II
separation of
sister chromatids
gametes
gametes
2n
zygote
diploid number
restored at
fertilization
Fig. 17.19, p. 366
Variation and genetic differences make us unique.
How is genetic diversity accomplished?
Evolutionary adaptation depends upon a
population’s genetic variation.
This is the basis for Darwin’s theory of evolution by
natural selection, which states that individuals with
certain traits may be better suited to survive and
reproduce than individuals with different traits.
There are two sources of genetic variation:
1. Mutations: random changes or relatively rare
mistakes made during DNA replication in a
gene.
2. Sexual reproduction (meiosis and fertilization)
1. Crossing-Over during Prophase I of Meiosis.
Multiple features of meiosis and fertilization
result in genetic diversity within a species.
1. Crossing over during prophase I of meiosis.
In each homologous pair (a complex of four chromatids),
nonsister chromatids are linked by X-shaped chiasmata,
sites where homologous strand exchange or crossingover occurs.
The exchange of genetic material between homologues
occurs during prophase of meiosis I.
This produces chromosomes that contain genes from both
homologues (e.g., a mixture of genes from both parents).
In humans, there is an average of two or three crossovers
per chromosome pair during meiosis.
The effect of
crossing-over
on generating
genetic
diversity
in gametes.
2. Independent Assortment
2. Independent Assortment
The random distribution of maternal and paternal homologues
to the gametes.
Since each homologous pair lines up randomly during
Metaphase I and assorts independently from all the others,
the process produces 2n possible combinations, where n is
the haploid number. In humans, there are 223 or ~8 million
possibilities of chromosome assortments in any gamete.
Genetic variation results from the reshuffling of chromosomes,
because each homologue will carry different genetic
information (alleles) at many of their corresponding loci.
3. Random fusion of gametes during fertilization.
In humans, when an individual ovum (representing
one of eight million combinations) is fertilized by a
sperm (representing one of eight million
combinations), the resulting zygote can have one of
64 trillion possible diploid combinations. This is not
even considering variations from crossing over!
germ cell
2n
germ cell
each chromosome
duplicating during
interphase
MEIOSIS I
separation of
homologues
n
MEIOSIS II
separation of
sister chromatids
gametes
gametes
2n
zygote
diploid number
restored at
fertilization
Fig. 17.19, p. 366