2. The Human Genome
• Genome: Complete
complement of an
organism’s DNA.
– Includes genes
(control traits) and
non-coding DNA
organized into
chromosomes.
3. Genes
• Eukaryotic DNA is organized in
chromosomes.
– Genes have specific places on
chromosomes.
4. Heredity
• Heredity – way of
transferring genetic
information to
offspring
• Chromosome theory
of heredity:
chromosomes carry
genes.
• Gene – “unit of
heredity”.
5. All sexually reproducing organisms have two
types of cells in their bodies
• Diploid cells
– Normal body cells
– Produced during
mitosis
– 2 sets of
chromosomes paired
together (2n)
• Haploid Cells
– Sex cells (gametes)
– Produced during
meiosis
– Only 1 set of
chromosomes (n)
6. Diploid cells
A normal diploid human body cell has 46
chromosomes paired together (23 pairs)
• The paired chromosomes match each other
in gene type and location
• They are called homologous pairs
8. Looking at Chromosomes in
a cell
• Karyotype:
– ordered
display of an
individual’s
chromosomes
– Chromosomes
are stained to
reveal visible
band patterns
and major
abnormalities.
11. • Trisomy - Extra chromosome so 3 instead of 2
• Monosomy - Missing a chromosome so 1 instead
of 2
Chromosomal disorders
Down’s
Syndrome
(trisomy 21)
12. Genes are passed on through
Reproduction
• Asexual
–Without sex cells
–Produces identical
copies of parent
(clones)
–Mitosis is asexual
reproduction
• Sexual
–With sex cells
–Produces
genetically
variable offspring
• This is good for
survival!
–Meiosis is sexual
reproduction
13. Asexual Reproduction
• single-celled organisms reproduce by
splitting, budding, parthenogenesis.
• offspring are genetically identical to parent.
14. Sexual reproduction
• Important vocabulary
– Gametes = sex cells (egg and sperm) that are
haploid
– Haploid = one set of chromosomes
– Diploid = two sets of chromosomes
– Zygote = fertilized egg
• Fusion of two gametes to produce a single
fertilized egg (zygote).
15. 15
Meiosis (sexual) vs. Mitosis
(asexual)
• Meiosis reduces the
number of
chromosomes by
half.
– Produces 4 haploid
cells
• Daughter cells differ
from parent cell
• Meiosis involves two
divisions
• Mitosis keeps the
same number of
chromosomes
– Produces 2 diploid
cells
• Daughter cells
identical to parent
(clones)
• Mitosis involves only
one division
16. Sexual Reproduction is
Important!
• Because a zygote has genes from two
different parents…..
–Introduces greater genetic variation for a
species
–Allows for genetic recombination
–Increase species survival!
17. 17
Meiosis I - First division of meiosis
• Interphase - all chromosomes replicate
(just like in mitosis)
• Prophase 1: Homologous chromosomes
begin to pair up. Crossing-over can occur
during the latter part of this stage.
18. 18
Crossing over - occurs during
Prophase I of meiosis
Genetic recombination that
is an exchange of genetic
material between
homologous chromosomes
Crossing over produces
recombinant
chromosomes and
increases genetic
variation!
20. 20
Meiosis I continued
• Metaphase 1: Homologous chromosomes
align at the equatorial plate. (in pairs next
to each other)
• Anaphase 1: Homologous pairs separate
with sister chromatids remaining together.
• Telophase 1: Two daughter cells are
formed with each daughter containing only
one chromosome of the homologous pair.
21. 21
Meiosis II
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate again.
• Metaphase 2: Chromosomes align at the
equatorial plate.
• Anaphase 2: Centromeres divide and sister
chromatids migrate separately to each pole.
• Telophase 2: Cell division is complete. Four
haploid daughter cells are obtained.
22. 22
Meiosis creates genetic variation
• Meiosis results in genetic variation by
shuffling of maternal and paternal
chromosomes and crossing over.
*No daughter cells formed during meiosis
are genetically identical to either mother or
father
*During sexual reproduction, fusion of the
unique haploid gametes produces truly unique
offspring.
24. 24
In humans
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
25. 25
Random fertilization
At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
26. 26
Meiosis & sexual life cycles
• Life cycle = sequence
of stages in organisms
reproductive history;
conception to
reproduction.
• Somatic cells = any
cell other than
gametes, most of the
cells in the body.
• Gametes produced by
meiosis.
Generalized animal life cycle
27. 27
Sex is costly!
• Large amounts of energy required to find a
mate and do the mating: specialized
structures and behavior required
• Intimate contact provides route for infection
by parasites (AIDS, syphillis, etc.)
• Genetic costs: in sex, we pass on only half of
genes to offspring.
• Males are an expensive luxury - in most
species they contribute little to rearing
offspring.
28. 28
But …
• More genetic diversity: more potential for survival
of species when environmental conditions
change.
– Shuffling of genes in meiosis
– Crossing-over in meiosis
– Fertilization: combines genes from 2 separate
individuals
• DNA back-up and repair.
– Asexual organisms don't have back-up copies of
genes, sexual organisms have 2 sets of chromosomes
and one can act as a back-up if the other is damaged.
– Sexual mechanisms, especially recombination, are
used to repair damaged DNA - the undamaged
chromosome acts as a template and eventually both
chromosomes end up with the correct gene.
29. 29
Study Questions
• 1. What happens as homologous
chromosomes pair up during prophase I
of meiosis?
• 2. How does metaphase of mitosis
differ from metaphase I of meiosis?
• 3. What is the sole purpose of
meiosis?
• 4. What specific activities, involving
DNA, occur during interphase prior to
both mitosis and meiosis?
30. 30
5. Compare mitosis and meiosis on the
following points:
• a. number of daughter cells produced.
• b. the amount of DNA in the daughter cells
in contrast to the original cell.
• c. mechanism for introducing genetic
variation.
• 6. What is a zygote and how is it formed?
• 7. What is the main advantage of sexual
reproduction?
31. Meiosis – key differences from
mitosis
• Meiosis reduces the number of
chromosomes by half (haploid cells).
• Daughter cells differ from parent cell and
each other.
• Meiosis involves two divisions, Mitosis
only one.
• Meiosis produces 4 haploid cells, Mitosis
produces 2 diploid cells.
