Download 3.1 Meiosis Notes (Key Facts)

3.1 Meiosis Notes (Key Facts)
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• Sexual Reproduction – is when two parents are needed to create an
offspring
• For sexual reproduction to occur, organisms need specialized cells to aid in
reproduction called gametes.
o Gametes : a gamete is a cell that fuses with another cell during
fertilization in organisms that reproduce sexually.
• Gametes can be broken down into sperm and egg
o Sperm : Male gametes - many, small, mobile
o Eggs : Female gametes - are relatively few, large, and do not move.
• To produce the gametes, specific body cells undergo a specific type of cell
division called meiosis. This produces a cell that is said to be haploid.
o Regular cells (called somatic cells) have their chromosomes arranged
in pairs called homologous chromosomes.
Homologous chromosomes:
Pairs of chromosomes are called homologous chromosomes.
Homologous chromosomes are the same size & shape and carry the same genes.
Note that they may carry different versions of the genes (called alleles).
• This split is needed because since sexual reproduction requires two parents,
if there isn’t a reduction of chromosomes, then the offspring would have too
many chromosomes in its cells.
• Steps to meiosis
o The steps of meiosis are similar to those of mitosis, except with a
couple of slight differences
Where meiosis goes on:
Testes & ovaries of animals
Anthers & ovules of flowering plants
Diploid v. Haploid
Diploid cells (2n) carry two copies of each chromosome (homologous pairs).
Haploid cells (n) carry one copy of each chromosome.
Human diploid cells have 46 chromosomes; haploid cells (gametes) have 23.
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Meiosis I: 1st Division
Interphase
• DNA is replicated
• Each chromosome duplicates to become 2 sister chromatids,
but they are loosely coiled, so not visible yet.
Prophase I
• Nuclear envelope disintegrates.
• Chromosome start to become visible because they coil,
shorten & thicken (condense).
• Centrioles (in animal cells) begin to make spindle fibres to
move the chromosomes around.
• Homologous chromosomes pair (synapsis) up.
• They swap portions of chromatid at crossing-over
points called chiasmata.
Metaphase I
• Bivalents move to the centre of the cell along protein
tubules called spindle fibres.
• They line up on the equator of the spindle fibres.
Anaphase I
• Spindle fibres contract and pull the chromosome pairs apart.
• the pairs ofhomologous chromosomes separate and move to
opposite poles along with their sister chromatids.
Telophase I
• Chromosomes arrive at the poles of the cell
• The cell divides into two
Meiosis I results in two daughter cells, but each has only one of the homologous
chromosomes, with its sister chromatid still attached.
Meiosis II: 2nd Division
Prophase II
• Chromosomes relax, then condense again
• A second set of spindle fibres forms at right angles
to the spindle fibres in the first division.
Metaphase II
• Chromosomes line up on equator of the spindle fibres.
• Note spindle fibres form at right angles to 1st division
Anaphase II
• Spindle fibres contract, centromeres split, &
chromatids are pulled apart
• Once pulled apart they are called chromosomes
Telophase II
• Chromosomes arrive at the poles of each cell
• Each cell divides into two
• Four sex cells (gametes) are made
Mitosis is the cell division process that occurs in our body and does not change
the number of chromosomes in the cell. It happens in all somatic cells of the body
and helps to make new cells.
In meiosis, the number of chromosomes is halved and therefore, it is also known
as Haploid Division or Reduction Division. It helps in variation in the offspring and
sex determination, as half the chromosomes are from the father and half from the
mother.
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