Human Genetics
Mitosis and Meiosis
Chromosomes and Cell Division
How are Chromosomes replicated?
Cell Division:
 Why are there two types: mitosis and meiosis?
 How are they different?
 How does meiosis produce new combinations
of genes?
What is a Chromosome?
It is a linear strand of DNA in combination
with nuclear proteins.
We call this complex of DNA and proteins
chromatin.
 It is a linear array of genes
 As a set - they are our genome
Chromosomes exist in different
states
As visible structures seen during cell
division.
Most of the time they are not visible
because they exist as uncoiled threads
of chromatin distributed through out the
nucleus.
Chromosome Basics
The number of chromosomes present within the
nucleus is a characteristic of the species.
Chromosomes of humans and most other
eukaryotic species occurs in pairs.
Members of a chromosome pair are known as
homologs.
Chromosomes in a pair are homologous
because they have the same genes (coding for
the same proteins) but not always the same
alleles.
Haploid vs. Diploid
 The state in which there are two copies of each
chromosome present is known as diploid.
Haploid – only one copy of a chromosome is
present (half of the diploid). This number is
referred to as n.
 All of the somatic cells in our bodies are diploid
cells. Therefore, they are 2n.
 The only haploid cells in our bodies are our
gametes – eggs and sperm.
Different organisms have different
chromosome numbers
Human somatic cells have 46 chromosomes
Diploid number (2n) = 46
23 from mom - 23 from dad
22 pairs are autosomes –true homologous pairs
1 pair is not necessarily homologous
- Sex chromosomes
Females are XX
Males are XY – a non homologous pair.
Chromosomes in the cell
Chromosome Replication
Chromosomes are different sizes
and shapes
Cell Division
Mitosis is the process of cell division in
which one cell becomes two identical
daughter cells.
It is required for
• development
• renewal
• regeneration
Cell Cycle
S phase: DNA
synthesis
G phase: gap for
growth
M phase: mitosis
(nuclear division)
Cell division or
cytokinesis is part of
G1
Chromosome Replication
 Replication is the
process of duplicating a
chromosome
• Occurs prior to division
• Replicated copies are
called sister chromatids
• Held together at centromere
The DNA is condensed for mitosis
Mitosis
Purpose: to make copies of cells and their DNA
• Replicated chromosomes align at the metaphase plate.
• Sister chromatids separate and move to opposite poles.
• Nuclear membranes form around each new nucleus.
• Division of cytoplasm or cytokinesis occurs.
Prophase
Replicated
chromosomes
condense.
Microtubules
organize into a
spindle
Metaphase
Chromosomes
line up on the
metaphase plate.
Spindle
microtubules are
attached to
centromeres of
chromosomes
Anaphase
Centromeres of
sister chromatids
separate
Chromosomes
move to opposite
ends of the cell
Telophase
Nuclear
membranes form
Spindle
disappears
Division of
cytoplasm occurs
(cytokinesis)
Cytokinesis
Cytoplasmic division
occurs after nuclear
division is complete.
Two cells are
formed.
Cells enter G1
Cell Cycle Control
Checkpoint proteins monitor progression through the cell cycle.
Cell death
Apoptosis is the process of cell death in
which cells signal their own demise in a
programmed way
Development
Response to abnormality
How Apoptosis Works
.
Signal molecule
binds to death
receptor.
Caspase enzymes
destroy proteins.
Cells fragment.
Phagocytes engulf
cell remnants.
Apoptosis is critical for regulation
of development in embryos.
Different
amounts of
apoptosis
create feet
with digits
versus
webbing.
Mitosis
A form of cell division that produces two
daughter cells, each having the same genetic
complement as the parent cell.
2n --- briefly 4n (anaphase) ------2n
Identical chromatids (sister chromatids)are
separated and then segregated to different
daughter cells.
Occurs in the somatic cells of our bodies
Meiosis
Specialized form of cell division with two
successive rounds of cell division following
DNA replication
Produces haploid cells (n)
Start with 46 double stranded chromosomes
(2n)
 After 1 division - 23 double stranded chromosomes (n)
 After 2nd division - 23 single stranded chromosomes (n)
Occurs in our germ cells
 cells that produce our gametes
 egg and sperm
Why do we need meiosis?
It is the fundamental basis of sex.
What is the purpose of sex?
- to bring two haploid gametes
together to form a diploid zygote.
n (mom) + n (dad) = 2n
(offspring)
Chromosome Replication
 Replication is the
process of duplicating a
chromosome
• Occurs prior to division
• Replicated copies are
called sister chromatids
• Held together at centromere
Why form gametes?
Goal: Reduce genetic material by half
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Meiosis:cell division in two parts
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid
Haploid
Haploid
Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction division
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Homologs separate in meiosis I and
therefore different alleles separate.
Gene X
homologs
sister
chromatids
same genes
maybe different
alleles
same genes
same alleles
Result: Reduction in genetics of cell
from two genomes from parents to one
random genome from both parents
Prophase I
Early prophase
Late prophase
Homologs pair.
Crossing over occurs.
Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Metaphase I
Homolog pairs align
along the equator of the cell.
Anaphase I
Homologs separate and
move to opposite poles.
Sister chromatids remain
Attached at their centromeres.
Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
Meiosis II:Only one homolog of each
chromosome is present in the cell.
Gene X
Sister chromatids carry
identical genetic
Information for gene X
Meiosis II produces gametes with
one copy of each chromosome and thus one
copy of each gene.
Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
nonidentical
haploid
daughter cells
Prophase II
Nuclear envelope fragments.
Spindle forms.
Metaphase II
Chromosomes align
along equator of cell.
Anaphase II
Sister chromatids separate
and move to opposite poles.
Telophase II
Nuclear envelope
assembles.
Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into
two cells.
Results of meiosis
• Gametes
• Four haploid cells
• One copy of each
chromosome
• One allele of each gene
• Different combinations
of alleles for different
genes along the
chromosome
Meioisis vs. Mitosis
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Germline cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction
Genetically
identical?
Chromosome #
Why do we have meiosis?
To generate haploid gametes!
To make new combinations of genes.
How?
Random (independent) assortment
Recombination
Independent assortment
The homolog of
one
chromosome
can be inherited
with either
homolog of a
second
chromosome.
Maternal and Paternal chromosomes
shuffle in your children
Independent assortment generates
variation in offspring
Since the combination of maternal and parental
chromosomes received by a gamete is
random;
And we have 23 pairs of chromosomes;
The possible combinations in an egg or a
sperm are:
223 = 8,388,608
combinations in an offspring
223 X 223 = 70,368,744,177,664
Result: New combinations of genes (alleles)
are generated when the genes are located on
different chromosomes.
Recombination generates variation
Crossing-over is:
 The physical exchange of chromosomal
material between chromatids of homologous
chromosomes.
Result: Generation of new combinations
of genes (alleles) if the genes are located
on the same chromosome.