Download Sister Chromatids

Introduction to Genetics:
One Cell Becomes Two:
Mitosis and Cytokinesis
Why do cells divide?

Growth

Repair/regeneration

Reproduction

asexual
DNA Is Packaged in
Chromosomes
.
1. DNA is divided into
long strands wrapped
around protein
(chromatin).
2. Each strand is
packaged and condensed
into a single chromosome.
Chromosomes
Usually Invisible - Only visible during
cell division (mitosis or meiosis)
 During Interphase – the DNA replicates
(makes an exact copy of itself)
 the cell has twice as much DNA in it after
replication
 Once a chromosome has replicated, it
shortens and thickens and becomes
visible
 See animation


When cells divide by mitosis, each
daughter cell receives the same
number of chromosomes as its
mother cell has – 2n.
46
46
46

In order to do this, the chromosomes must
be copied first, then one of each copy is
placed in the new cells.
46
46
92
46
One Chromosome
Sister
Chromatids

Each strand is an
identical copy
of the other one
DNA
Centromere
Where the two
chromatids
Are attached to
each other –
DNA is the key to reproduction, development, and
maintenance.
1. Genome = complete collection of an
organism’s genetic information as linked
genes in a long strand of DNA.
2. Information is found in letters A, C, G,
and T in the double helix
3. Humans have about 100,000 genes that
have all the information to make all
the proteins (especially enzymes) a
cell needs.
4.
Replication takes one chromosome and
makes two identical copies, called sister
chromatids
Chromosome Number

Each species has the same number of chromosomes in all
their cells that are made by mitosis. This is the diploid
number (2n). In humans this number is 46. So cells of your
skin and muscle and liver each have 46 chromosomes in
them. Look how many chromosomes are in the cells of these
creatures:
2n = 42
2n = 78
2n = 38
2n = 94
Sex Chromosomes

Homologous in females: XX

Not homologous in males: XY
A male karyotype: 22 pairs of homologous
chromosomes; one pair of sex chromosomes
The cell cycle keeps record of progress of a cell
over time, like a clock
1. The cell cycle is made up of a repeating
pattern of growth, genetic duplication, and
division.
2. Typical animal cell cycle lasts about 24 hours.
3. Two main phases: interphase and mitotic phase
(about 30 minutes).
4. Interphase = G1 (gap 1 for growth, 12 hours) + S
phase (synthesis, for replication of DNA, 6
hours) + G2 (gap 2, 6 hours)
Mitosis




Nuclear division resulting in nuclei
identical to parent cell –
Begins after interphase
Ends before cytokinesis
Four phases: Prophase
Metaphase
Anaphase
Telophase
1. Prophase (P for “plain to see”)—Chromosomes
condense, nuclear envelope breaks down,
formation of spindle fibers (microtubules) from
the centrosomes.
2. Metaphase (M for “middle”)—Chromosomes are
aligned on the equator by pushing along spindle
with each sister chromatid facing opposite
poles.
3. Anaphase (A for “apart”)—Sister chromatids
separate; each new chromosome moves to the
opposite pole.
4. Telophase (T for “two nuclei”)—Chromosomes decondense, spindle breaks down, nuclear envelope
forms around the two separate complements of
chromosomes.
Mother cell
Prophase
Prophase
Prophase





Chromosomes become visible
Spindle forms from protein microtubules
Nuclear envelope disintegrates
Nucleolus disintegrates
In animal cells, centrioles migrate to
opposite ends of the cell (poles) and
spindle fibers attach to them
Metaphase

Chromosomes line up single file at
the equator of the cell
Anaphase


Sister chromatids are pulled apart
toward opposite poles
In animal cell, cleavage furrow
begins to form
Telophase





Nuclear membrane forms around each group of
chromosomes
Nucleolus reappears in each nucleus
Spindle fibers disappear
Chromosomes become invisible again as
chromatin
Cytokinesis begins in plant cell by formation of
cell plate; cleavage furrow in animal cell
completely separates the two nuclei into two
different cells.
Cytokinesis
Occurs after nucleus has been duplicated
Begins in anaphase in animal cells
by the formation of a cleavage furrow
Begins in telophase in plant cells
by the formation of a
cell plate.
V.Variations in Cell Division
A.Plant cells—Everything is
similar except for
cytokinesis because plant
cells have to break down
and reform the cell wall
1.Vesicles fuse near the
metaphase plate for
form a cell plate that
grows outward to form
a cell wall.
Find the different stages of mitosis in these onion
cells:
Meiosis
Cell division producing cells that
have half the number of
chromosomes of the mother cell
 Produces gametes – eggs and sperm
 Occurs so that fertilization
doesn’t increase the number of
chromosomes in each generation.

Gametes = sex cells



Eggs or sperm
Have half the normal number of
chromosomes = haploid (n) = 23 in humans
Sexual reproduction needs these to
combine DNA from two different
parents, producing offspring that is
different from each parent
Meiosis = Reduction Division
Two complete cell divisions
 First cell division – separates
homologous chromosomes (reduction
of chromosome number)
 Second cell division – separates sister
chromatids (like mitosis) - Division
 Produces 4 haploid cells

M
M
E
E
I
I
O
O
S
S
I
I
S
S
Crossing over can occur
I
Homologous pairs are separated
Sister chromatids are separated
II
Gametogenesis

Oogenesis
Spermatogenesis
Production of
sperm

Production of
an egg
One mother
cell produces
One mother
one egg cell
cell produces
and three
4 equally
polar bodies
that die
sized sperm
cells
2 things that cause variation within
a population
1. Crossing Over: When DNA or
genes are exchanged during
Metaphase 1
2. Independent assortment:
Chromosomes segregates
independently. This makes the
genetic make-up of a gamete
random
Fertilization
The sperm unites with the egg forming a
zygote (fertilized egg).
The zygote then divides by mitosis to
produce the trillions of cells that make up a
multicellular body like yours.
B. Prokaryotes (no nucleus) binary fission
III. When Cell Division Runs Amok:
Cancer
A. Unrestrained cell division—
cancer
1. Mechanisms that induce cell
division can become
hyperactive. (carcinogens)
2. Mechanisms that suppress cell
division can fail. (carcinogens)
B. Genes
1. Oncogenes: stuck accelerator
2. Tumor suppressor genes: failed
brakes
Skin cancer
The End