Download Chromosomes and Cell Division

Chapter 6: Chromosomes
and Cell Division
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Continuity and variety
Lectures by Mark Manteuffel, St. Louis Community College
6.4 There is a time for everything in the
eukaryotic cell cycle.
Take-home message 6.4
 Eukaryotic
somatic cells alternate in a
cycle between cell division and other cell
activities.
 The
cell division portion of the cycle is
called the mitotic phase.
Take-home message 6.4
 The
remainder of the cell cycle, called
interphase, consists of two gap phases
(during which cell growth and other
metabolic activities occur) separated by a
DNA synthesis phase during which the
genetic material is replicated.
6.5 Cell division is preceded by
replication.
Persistence and propagation
Replication
 The
process of DNA duplication
Complementarity
 The
characteristic that in the doublestranded DNA molecule the base on one
strand always has the same pairingpartner (called the complementary
base) on the other strand
Complementarity
 Every
“A” (adenine) pairs with “T”
(thymine) and vice-versa.
 Every
“G” (guanine) pairs with “C”
(cytosine) and vice-versa.
Errors sometime occur when
DNA duplicates itself.
Why might that be a good thing?
Mutation
A
variety of errors can occur during
replication.
 Several
DNA repair processes occur after
replication.
 If
an error remains, however, the
sequences in a replicated DNA molecule
(including the genes) can be different
from those in the parent molecule.
Take-home message 6.5
 Every
time a cell divides, the cell’s DNA
must duplicate itself so that both new cells
have all the DNA of the parent cell.
 This
process of DNA duplication is called
replication.
 Errors
in replication can lead to changes in
the DNA sequence called mutations.
6.6–6.9
Mitosis replaces
worn-out old cells
with fresh new
duplicates.
6.6 Most cells are not immortal:
Mitosis generates replacements.
What is dust?
Why is it your fault?
Mitosis has just one purpose:
 To
enable cells to generate new,
genetically identical cells.
 There
need:
are two different reasons for this
1. Growth
2. Replacement
6.7 Overview
Mitosis leads to duplicate cells.
Parent cells  daughter cells
Take-home message 6.7
 Mitosis
is the process by which cells
duplicate themselves.
 Mitosis
follows chromosome replication
and leads to the production of two
daughter cells from one parent cell.
6.8 The Details
Mitosis is a four-step process.
Preparation for
Mitosis: The
Chromosomes
Replicate
Animal chromosomes are linear.
So why do they look like the letter
“X” in pictures?
Sister Chromatids
A chromosome and its identical replicated copy,
joined at the centromere.
Take-home message 6.8
 The
ultimate result of mitosis and
cytokinesis is the production of two
genetically identical cells.
6.10–6.14
Meiosis generates
sperm and eggs and
a great deal of
variation.
6-10 Sexual
reproduction
requires special
cells made by
meiosis.
Meiosis
 Gametes-
sex cells (haploid)
 Diploid
– 2n (2 full sets of chromosomes)1 set from each parent
 Haploid
–n (1 full set of chromosomes)[mother gives one set and father gives
one set]
 Maintains
species
a stable genome size in a
 Meiosis
achieves more than just a
reduction in the amount of genetic
material in gametes.
 You
have two copies of every gene!
Meiosis has two important features:
1. It reduces the amount of genetic
material in gametes.
2. It produces gametes that all differ from
each other with respect to the
combinations of alleles they carry.
Take-home message 6.10
 In
sexually reproducing organisms,
gametes are produced through meiosis.
 Gametes
have half as much genetic
material as the parent cell.
 Gametes
alleles.
carry different combinations of
6.11 Sperm and egg are
produced by meiosis: the
details, step-by-step.
Mitosis occurs almost
everywhere in an animal’s body.
Meiosis only occurs in one place.
Where?
Meiosis starts with a diploid cell.
 One
of the specialized diploid cells in the
gonads
Meiosis starts with a diploid cell.
A
homologous pair, or homologues
• The maternal and paternal copies of a chromosome
Chromosomes are duplicated.
 Sister
chromatids
• Each strand and its identical duplicate, held
together at the centromere
Cells undergoing meiosis divide twice.
There are two major parts to meiosis:
1. The homologues are separated.
2. Each of the two new cells divides
again, separating the sister chromatids
into two even newer cells.
Meiosis Division 1
Separating the homologues
1. Prophase I
 The
most
complex of all of
the phases of
meiosis
 Crossing
over
2. Metaphase I
 Each
pair of
homologous
chromosomes
moves to the
equator of
the cell.
3. Anaphase I
 Beginning
of the first cell division that
occurs during meiosis
 The
homologues are pulled apart toward
opposite sides of the cell.
 The
maternal and paternal sister
chromatids are pulled to the ends of the
cell in a random fashion.
3. Anaphase I
4. Telophase I and Cytokinesis
 This
phase is marked by the chromosomes
arriving at the two poles of the cell.
 The
cytoplasm then divides and the cell
membrane pinches the cell into two
daughter cells.
4. Telophase I and Cytokinesis
Meiosis Division 2
Separating the sister
chromatids
5. Prophase II
 The
genetic material once again coils
tightly making the chromatids visible
under the microscope.
 It
is important to note that in the brief
interphase prior to prophase II, there is no
replication of any of the chromosomes.
6. Metaphase II
 The
sister chromatids (each appearing as
an X) move to the center of the cell.
7. Anaphase II
 The
fibers attached to the centromere
begin pulling each chromatid in the sister
chromatid pair toward opposite ends of
each daughter cell.
8. Telophase II
 The
cytoplasm
then divides, the
cell membrane
pinches the cell
into two new
daughter cells,
and the process
comes to a close.
Outcome of Meiosis
 The
creation of four haploid daughter
cells, each with just one set of
chromosomes which contains a completely
unique combination of traits
Take-home message 6.11
 Meiosis
cells.
 It
occurs only in gamete-producing
occurs after DNA replication and
consists of two rounds of cellular division.
Take-home message 6.11
 In
the first round, homologous pairs of
sister chromatids separate and in the
second round, sister chromatids separate.
 The
final product of meiosis in a diploid
organism is four haploid gametes.
6.12 Male and female
gametes are produced in
slightly different ways.
How do you distinguish a
male from a female?
Take-home message 6.12
 In
species with two sexes, females
produce the larger gamete and males
produce a smaller gamete.
 Male
and female gametes both end up
with just one copy of each chromosome.
6.13 Crossing over and meiosis
are important sources of
variation.
Take-home message 6.13
 Although
it doesn’t create any new traits,
crossing over creates gametes with unique
collections of traits.
 This
variation is important for evolution.