Download The Process of Meiosis

Recognizing the
significance of
meiosis to
sexual
reproduction
Meiosis - The Genetics of Reproduction
Genetics and Evolution
The genetic information found in DNA
is essential in creating all the
characteristics of an organism.
This remains the case when passing
genetic information to offspring
that can occur via a process called
meiosis where four haploid cells
are created from their diploid
parent cell.
For a species to survive, and genetic
information to be preserved and
passed on, reproduction must
occur. This can be done by passing
on the information found in the
chromosomes via the gametes
that are created in meiosis.
Chromosome Complement
• Humans are diploid
creatures, meaning that
each of the chromosomes
in our body are paired up
with another.
• Haploid cells possess only
one set of a chromosome.
For example, a diploid
human cell possesses 46
chromosomes and a
gamete (Sex Cell) created
by a human is haploid, it
possesses 23
chromosomes.
Reproduction
•Reproduction occurs in humans with the fusion
of two haploid cells (gametes) that create a
zygote.
•The nuclei of both these cells fuse, bringing
together half the genetic information from
the parents into one new cell that is now
genetically different from both its parents.
•This increases genetic diversity, as half of the
genetic content from each of the parents brings
about unique offspring, which possesses a unique
genome presenting unique characteristics.
•Meiosis as a process can increase genetic
variation in many ways.
Chromosome Structure
A chromosome is an
organized structure of
DNA and protein found in
cells. It is a single piece of
coiled DNA containing
many genes, regulatory
elements and other
nucleotide sequences.
Chromosomes also contain
DNA-bound proteins,
which serve to package
the DNA and control its
functions.
Homologous Chromosomes
Each chromosome has a
match, called a
homolog. This is why
normal
organismsalways have
an even number of
chromosomes. One
homolog you received
from your mother, the
other you received
from your father. They
are not exactly alike,
but they are the same
size, shape, and have
the same banding
pattern.
Sex Chromosomes
•
In humans, there are two chromosomes that determine sex.
The X and the Y chromosome.
•
Reproduction occurs in humans with the fusion of two haploid
cells (gametes) that create a zygote. The nuclei of both these
cells fuse, bringing together half the genetic information from
the parents into one new cell that is now genetically different
from both its parents.
•
This increases genetic diversity, as half of the genetic content
from each of the parents brings about unique offspring, which
possesses a unique genome presenting unique characteristics.
•
Meiosis as a process can increase genetic variation in many
ways.
•
•
If you have an XX - you are female
If you have an XY - you are male
Diploid vs Haploid
• Body cells have the full set of chromosomes – they are DIPLOID
(In humans, 46)
• Sex cells (sperm and eggs) have half a set – they are HAPLOID
(In humans, 23)
• GAMETOGENESIS - process of creating gametes
(Spermatogenesis & Oogenesis)
The Process of Meiosis
The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell)
dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases
which are a continuous process.
1st Division
• Prophase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into
chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic
variation.
• Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up
is at random, through chance, increasing genetic variation via independent assortment.
• Anaphase - The homologous chromosomes move to opposing poles from the equator
• Telophase - A new nuclei forms near each pole alongside its new chromosome compliment.
At this stage two haploid cells have been created from the original diploid cell of the parent.
2nd Division
• Prophase II - The nuclear membrane disappears and the second meiotic division is initiated.
• Metaphase II - Pairs of chromatids line up at the equator
• Anaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibres
• Telophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a
zygote.
Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the
family tree and the species as a whole. Each of these gametes possesses unique genetic information due to
situations in meiosis where genetic diversity is increased.
Key points of Meiosis
• The process results in 4 daughter cells.
• Daughter cells are haploid (N).
• Daughter cells have unique combinations of
chromosomes.
• Daughter cells do not have homologous pairs.
• Meiosis creates gametes (sperm and eggs).
• Meiosis ensures variability in offspring.
• Gametes combine to create a zygote which is
diploid (2N) - process of sexual reproduction.
Independent Assortment and Crossing Over - Genetics and Evolution
• The process of meiosis results in 4 haploid gametes
created from the parent cell. Half the genetic
information from a parent is present in these
haploids, which fuse with gametes of the opposite
sex to create a zygote, with a complete chromosome
compliment that will create offspring after prolonged
growth.
• The process of meiosis increases genetic diversity in
a species. The sex organs which produce the haploid
gametes are the site of many occurrences where
genetic information is exchanged or manipulated.
Independent Assortment of
Chromosomes
• Alleles for a particular phenotype determine what characteristic an
organism will express, as with the following example
• Mendel’s law of independent assortment applies only to traits carried on
different chromosomes, i.e. unlinked genes
• Independent assortment occurs as a result of the alignment of homologs
during metaphase I, determining which maternal and paternal
chromosomes assort to each daughter cell
• Each pair of alleles separates independently of every other pair of unlinked
alleles.
• The previous diagram indicates that even
though the two homologous chromosomes
contain the same genetic information, the
assortment of the chromosomes (the order
they lie in) can determine what genetic
information is present in each of the 4
gametes produced. With 23 chromosomes in a
human gamete, their are 223 combinations
(8388608 combinations).
Crossing Over
• During meiosis, when
homologous chromosomes
are paired together, there
are points along
thechromosomes that
make contact with the
other pair. This point of
contact is deemed the
chiasmata, andcan allow
the exchange of genetic
information between
chromosomes. This further
increases geneticvariation.
• There are also many other
ways in which genetic
variation is increased in a
species gene pool, all of
which are described in the
following pages.