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Transcript
Topic 4 Genes, Chromosomes


Offspring acquire genes from
parents by inheriting
chromosomes.
Each gene in an organism’s
DNA exists at a specific locus
on a certain chromosome.
We inherit one set of
chromosomes from our
mother and one set from our
father.
Comparison of Asexual and Sexual
Reproduction


In asexual reproduction,
a single parent produces
genetically identical
offspring by mitosis.
Sexual reproduction
combines sets of genes
from two different
parents, forming
genetically diverse
offspring.
Fertilization and Meiosis alternate
in sexual life cycles

Sets of chromosomes—
Normal human somatic cells
are diploid. They have 46
chromosomes made up of two
sets of 23. --- one set from
each parent. There are 22
pairs of autosomes, each with
a maternal and paternal
homolog. The 23rd pair , the
sex chromosomes, determine
gender (XX) or (XY).
Behavior of chromosome sets in
the Human Life Cycle

At sexual maturity, ovaries
and testes (the gonads)
produce haploid gamete by
meiosis, each gamete
containing a single set of 23
chromosomes (n=23). During
fertilization, an egg and
sperm unite, forming a diploid
(2n = 46) single celled zygote,
which develops into a
multicellular organism by
mitosis.
Meiosis reduces the number of
chromosome sets from diploid to
haploid

The stages of meiosis
produce four haploid
daughter cells. The
number of chromosome
sets is reduced from two
(diploid) to one (haploid)
during meiosis I, the
reductional division.
A comparison of Mitosis and
Meiosis




Meiosis is distinguished from
mitosis by three events of
meiosis I
Prophase I: each homologous
pair undergoes synapsis and
crossing over.
Metaphase I: Chromosomes
line up as homologous pairs
on metaphase plate.
Anaphase I: Homologs
separate from each other;
sister chromatids remain
joined at the cetromere.
Meiosis II What’s left?

Separation of sister
chromatids.
Genetic variation produced in
sexual life cycles contributes to
evolution!

Three events in sexual
reproduction contribute
to genetic variation in a
population
Independent assortment of
chromosomes during meiosis.

Homologous
chromosomes can line
up in no particular order
of paternal or maternal
chromosomes.
Crossing over during meiosis I

In prophase I of meiosis I,
the replicated homologous
pair of chromosomes
comes together in the
process called synapsis,
and sections of the
chromosomes are
exchanged. You can see
that after crossing over,
the resulting
chromosomes are neither
entirely maternal nor
entirely paternal, but
contain genes from both
parents. Synapsis and
crossing over occur only
Random fertilization of egg cells by
sperm.
1.
During random
fertilization, any one of
the 8,388,608 possible
combinations of
gametes .... give rise to
sperm cells, which will
fertilize an egg and
result in the
offspring.
Chiasmata

Do to sister chromatid
cohesion, crossing over
leads to chiasmata,
which hold homologs
together.
Evolutionary Significance of
Genetic Variation Within
Populations.



Genetic variation is the raw
material for evolution by
natural selection.
Mutations are the original
source of this variation.
The production of new
combinations of variant genes
in sexual reproduction
generates additional genetic
diversity.
Mendel’s Law of Independent
Assortment


States that when gametes are
formed, the separation of one
pair of alleles between the
daughter cells is independent
of the separation of another
pair of alleles.
One allele does not follow
another when it is passed on
to a gamete—they will sort
independently.
Independent Assortment and
Meiosis


Why do traits get passed
on independently of one
another?
1. The orientation of
bivalents during
metaphase 1
Compare Mitosis to Meiosis
Property
Mitosis
Meiosis
DNA replication
Occurs during
interphase before
mitosis
Occurs during
interphase before
meiosis I begins
Number of divisions
One, including
prophase,
metaphase,
anaphase and
telophase
Two, each including
PMAT
Synapsis of
homologous
chromosomes
Does not occur
Occurs during
prophase I with
crossing over
Property
Mitosis
Meiosis
Number of
daughter cells
and genetic
composition
Two, each diploid
(2n) and
genetically
identical to the
parent cell
Four, each haploid (n)
, containing half as
many chromosomes
as the parent cell;
genetically different
from the parent cell
and from each other
Role in the body
Enables multicellular
adult to arise from a
zygote, produces
cells for growth,
repair, and in some
species, asexual
reproduction
Produces gametes,
reduces number of
chromosomes by half,
and introduces
genetic variability
among the gametes