Meiosis & Sexual Reproduction
• In sexually reproducing organisms, meiosis is the type of nuclear division
that reduces the chromosome number from the diploid (2n) number to
the haploid (n) number.
• Diploid number (2n) = total number of chromosomes in a regular cell of
diploid organism.
Onion has 2n = 16
• Haploid number (n) = half of diploid number.
Rice has 2n = 24
Maize has 2n = 20
• e.g. Humans have a diploid number (2n) = 46; human gametes (eggs and
sperms) have haploid (n) number of chromosomes = 23.
Food for thought: What do you think will happen in each generation
of sexually reproducing organism if the chromosome number of
gametes are not reduced to haploid?
Importance of meiosis for plant and animal
 Ensures that chromosome number is constant from generation to
generation by halving the diploid number (2n) to haploid (n) in gametes.
 It promotes genetic variation in each generation through crossing-over
and independent assortment of homologous chromosomes.
Homologous chromosomes
• Chromosomes occur in pairs in diploid
body cells. Members of each pair are
called homologous chromosomes or
homologues.
• The homologues are held together by
Cohesin proteins, which form a
Synaptonemal complex. The process is
called Synapsis, which is very essential.
Why do chromosome occur in pairs?
An overview of Meiosis
Stages of Meiosis
• The cross-over can also be termed genetic recombination.
• The site where cross-over occurs is called Chiasmata.
• The number of possible chromosome orientation equals 2 to the power
of the chromosome pairs. This is called independent assortment. E.g. 22
How Meiosis promotes genetic variation
1. Genetic Recombination
2. Independent Assortment
Genetic Recombination:
• During meiosis I when homologous chromosomes pair to form
bivalents, crossing-over usually takes place.
• Leads to exchange of genetic materials between non-sister
chromatids.
•
• The offspring can thus have different sets of genes from their
parents.
Independent assortment:
• Homologous chromosome pairs separate independently
(i.e. in a random manner).
• When homologues align at the metaphase plate in
meiosis I, any member of a pair may be oriented towards
either pole.
• A cell with 3 pairs of homologous chromosomes has a
possible 23 (i.e. 8) combinations of maternal and paternal
chromosomes in the gametes from this cell simply due to
independent assortment of homologues.
Importance of genetic variation
 Increases genetic variation in the population
 If the environment does not change, asexual reproduction may be
good enough. However in a changing environment, genetic variability
among offspring may be advantageous.
 Under new conditions, some offspring, because of their genetic
differences, may have a better chance of survival and reproductive
success than others.
 Genetic variation is like an evolutionary insurance premium: you
never know when it might come in handy!
Comparison between Mitosis and meiosis
Mitosis
Requires one nuclear division
Produces two daughter cells
Daughter cells have the same number
of chromosome as parent cell
Meiosis
Requires two nuclear divisions
Produces four daughter cells
Daughter cells are haploid i.e. have half the
number of chromosomes in the parent cell.
Daughter cells are genetically identical Daughter cells are neither genetically identical
to each other and to parent cell
to each other or to the parent cell
Occurs in most body tissue during
growth and repairs
Occurs only sexually reproducing organisms and
only in reproductive organs to produce gametes
No bivalents formation or crossingover
Bivalents are formed and crossing-over is
possible
Can you reflect?
• Distinguish between haploid and diploid cells, and define homologous
chromosomes.
• Explain significance of Meiosis and describe the process.
• Contrast Mitosis and Meiosis.