Transcript Meiosis Notes

Meiosis
The Making of Gametes
Sea Urchin Gametes
Human Gametes
 Meiosis is a process
of reduction division
 The number of
chromosomes per
cell is cut in half
 This creates 4
haploid daughter
cells
Homologous Chromosomes
 A chromosome pairs
with another
chromosome during
meiosis
 This pairing
(synapsis) happens
between two
chromosomes that
are homologous
 Homologous chromosomes have the
same genes at the same loci but
possibly different alleles
 For example, two chromosomes may
have genes encoding eye color, but
one may code for brown eyes, the
other for blue.
 Loci – (singular – locus) specific location of a
gene on a chromosome
 Gene – a segment of DNA that codes for a trait
 Allele – different forms of a gene (dominant-recessive)
 A human cell contains 23 pairs of
homologous chromosomes:
 22 of them are homologous non-sex
chromosomes (or autosomes)
 1 pair are homologous sex chromosomes
 XX in females
 XY in male
Phases of Meiosis
 Four haploid daughter cells are produced
 Each have half the number of chromosomes
as the parent cell
 Meiosis allows for rapid generation
of new genetic combinations
 Three factors contribute to this
genetic variation:
 Independent Assortment
 Crossing over
 Random fertilization
 Independent Assortment – Chance
determines which one of the two parental
chromosomes is passed on to the gamete.
 Crossing over – DNA exchange
occurs during this process which
leads to even more recombination of
the chromosomes
 Random Fertilization – the zygote that
forms a new individual is created by
the random joining of two gametes
possible outcomes 223 X 223 = 64 trillion
Importance of Genetic Variation
 The pace of evolution appears to increase as
the level of genetic variation increases
•Racehorse breeding – choosing individuals
for speed started decades ago, most genetic
recombinations have been used
•As a result winning times in major races
stopped dramatically improving decades ago