Meiosis
Passing of Genetic information from one generation to the other
In sexual reproduction gametes fuse together to form a zygote
*fertilization
Male
Female
Sperm
(gametes)
Ova (egg)
Zygote
Embryo
Fetus
Baby
All humans have the same number of chromosomes in all their body cells
(somatic cells) – 46 or 23 pairs
- this number of chromosomes represents the diploid number of
chromosomes
- dogs have 78, gold fish have 100, fruit flies have 8
PROBLEM: What would happen if gametes also had the diploid number of
chromosomes?
46
+
46
=
92
-
23
-
this would result in a zygote with 92 chromosomes which is not human
gametes, therefore, must have half the number of chromosomes or
the haploid number so that fertilization results in the normal number
of chromosomes
+
23
=
46
there must be a process that allows for the formation of gametes
with the haploid number of chromosomes
this process is called meiosis; this process occurs only in the gonads
(testes for males, and ovaries for females)
Homologous Chromosomes
- Of the 46 chromosomes in each human cell, only 23 are distinctly
different from each other – the other 23 are duplicates. This means
that each chromosome has an identical partner that is alike in shape and
size. These duplicates also carry the same genetic information, such as
hair colour, thumb shape, etc.
- called homologous chromosomes or homologs
- offspring receive 1 maternal homolog from their mother and 1
paternal homolog from their father
- humans have 23 pairs of homologs
-
-
the diploid number of chromosomes can be represented in the form
2N which means 2 sets of the genetic information (in humans – 2(23) =
46)
the haploid number can be represented as N (in humans – N=23)
Meiosis = formation of sperm and ova
- different from mitosis in that the daughter cells produced have half
the number of chromosomes as the mother cell
- similar to mitosis in that there are the same stages
- different in that there are two sets of these stages
- similar in that both processes start with doubling of the chromosomes
during interphase
1st Meiotic Division (also called reduction division)
Prophase 1:
- same as mitosis except that homologous chromosomes find each other
and become intertwined – called synapsis
- during synapsis non-sister chromatids exchange pieces of genetic
information – called crossing-over
- this crossing-over allows genetic information from the father to mix
with genetic information from the mother – thus increasing genetic
variation within the species
Metaphase 1:
- chromosomes line up along the equator of the cell but unlike mitosis,
homologous chromosomes must line up together
Anaphase 1:
- homologous chromosomes are separated from each other as they are
pulled towards the poles of the cell
- this separation of homologs is called segregation
o each homolog is still a chromatid pair unlike in mitosis where, at
this stage, the chromatids separate
Telophase 1:
- two cells result at this point but instead of being exact copies of the
mother cell as in mitosis, these two cells have only one of the
homologous chromosomes and this is still make up of unseparated
chromatids
Short Interphase occurs – cells rest
Prophase 2:
- same as in mitosis
Metaphase 2:
- chromatid pairs line up at equator
Anaphase 2:
- chromatid pairs are pulled apart
Telophase 2:
- 4 cells result, each with the haploid number of chromosomes
o in the formation of eggs three of these cells are small and
useless (called polar bodies) because they didn’t get an equal
share of cytoplasm during cytokinesis, so only one egg is formed
o in spermatogenesis (formation of sperm) all four cells are viable
sperm cells capable of fertilizing an egg.