Download Cell Division

Cell Cycle and Cell
Division
Cell Division
• Creates two identical “daughter” cells
roughly ½ the size of the parent cell
• Before the cell divides:
– DNA of the cell (chromosomes) must be
copied in interphase
– chromosomes must be separated in
mitosis
• Actual cell division is cytokinesis.
Why do cells have to divide
• Surface area/volume ratio
• Too much demand placed on DNA
Larger the cell, more proteins need
to be made!
Only one copy of each DNA
molecule in a cell!
• Signals from outside the cell
Cell Division
• In multicellular organisms, the somatic
(body cells) cells undergo mitosis.
• Germ cells (cells destined to become
sperm eggs) divide by a related process
called meiosis.
• Prokaryotic cells (no nucleus) divide by
a process called binary fission.
Forms of DNA
• DNA + Proteins = Chromosomes
• Most of the time, DNA is “unspooled”
into loose strands – called chromatin
– can be used to provide instructions in this
form.
• Before a cell divides, the DNA winds
around histone proteins and becomes
visible as “chromosomes”
• The chromosomes can be counted in this
form.
Structure of
DNA
Chromosome
Nucleosome
DNA
double
Coils
Supercoils
Histones
helix
Chromosome number
• Eukaryotic organisms have a specific
number of different types of
chromosomes.
• They have two of each type – so
chromosomes come in pairs.
• Cells with chromosomes present in
pairs are said to be diploid (2N)
– one of each is haploid
Chromosome identity
• Each chromosome type can be
identified by shape and size.
• If stained, characteristic patterns of
bands can be seen as well.
• A karyotype is a display of all of an
individual’s chromosomes arranged by
type.
– Can be used to identify major genetic
disorders.
Chromosome number
• Each organism has a different number of
chromosomes:
Camel 70
Chicken 78
Opossum 22
Housefly 22
Bat 44
Corn 24
Lentil 14
Rice 24
Goat 60
Barley 14
Apple 34
Lettuce 12
HUMANS HAVE 46! 23 different pairs
How many chromosomes?
• In a diploid cell, one set of chromosomes
comes from the mother and another from
the father.
• Each human gets 23 from each parent
(46 total)
– 44 are autosomes (general body
characteristics) and two are sex
chromosomes (determine sex and carry
general characteristics)
Being Diploid
• The two corresponding chromosomes are
called homologous chromosomes.
Homologous chromosomes need not be
genetically identical.
– For example, a gene for eye color at one
locus (location) on the father chromosome
may code for green eyes, while the same
locus on the mother chromosome may code
for brown.
Before Cell Division, each
chromosome must replicate!
• Individual
chromosomes
replicate and form
sister chromatids.
• Each sister
chromatid is
destined for one of
the two resulting
daughter cells
• Sister chromatids
together are
considered one
chromosome.
• After separation, the
two independent
copies are sister
chromosomes.
• Each sister
chromosome goes to
a different cell.
After Replication  Chromosomes
The Cell Cycle
• Sequence of events a
cell goes through as it
grows and divides
• (G1 Phase) Cell grows
and synthesizes proteins
and new organelles
• (S Phase)
Chromosomes replicate
• (G2 Phase) Organelles
and molecules used in
cell division are
produced
• (M Phase) mitosis
(chromosomes) and
cytokinesis (cytoplasm)
are divided.
Cell Division M-Phase
• Consists of TWO steps (Mitosis and
Cytokinesis)
• Mitosis  process by which a cell separates its
duplicated genome into two identical halves.
Mitosis only separates the newly replicated
chromosomes; DNA replication does not
occur during mitosis.
– broken down into five phases: (PMAT)
Prophase, Metaphase, Anaphase, Telophase.
• Cytokinesis which divides the cytoplasm and
cell membrane.
Mitosis  Prophase
• Longest phase of mitosis
• Chromosomes condense
(become visible)
• Centrioles (in cytoplasm)
separate and move to
opposite sides of cell
• Nuclear membrane breaksdown
• Microtubule structure
called the spindle develops
(attaches from centrioles to
chromosomes
Chromosome Structure
• Prior to separation,
the two sister
chromatids are
attached together
in a specialized
region of the
chromosome
known as the
centromere.
Mitosis  Metaphase
• Chromosomes lineup along center of
cell (metaphase
plate)
• Each chromosome is
connected to its
centromere by a
spindle fiber
Mitosis  Anaphase
• Sister chromatids
separate into
separate
chromosomes
• Separated
chromosomes
pulled to opposite
sides
Mitosis  Telophase
• Chromosomes
move together at
opposite ends of the
cell and become
less condensed
• Spindle breaks apart
• Two new nuclear
membrane form
• Result is one cell
with 2 nuclei!
Cytokinesis
• Remember, NOT part of mitosis
• Animals
– Cell membrane pinches off
cytoplasm into two equal parts at a
region called the cleavage furrow
• Plants
– Cell Plate develops between two
new nuclei which grows into a
separating membrane and
ultimately a separating cell wall
Mitosis Animation
Cell Cycle Animation
• http://www.cellsalive.com/mitosis.htm
Limits to Division?
• Problem with eukaryotes is that they have to
replicate linear chromosomes. The polymerase
enzyme can’t work all the way to the end, so the
chromosome gets shorter with each round of
replication.
• Solution: use special ends called telomeres that
don’t contain genes. “Expendable” DNA.
• When cell runs out of telomere, can’t divide any
more.
• May act as “replication counter” for cell.
Regulation of Cell Cycle
• Not all cells move through cell cycle at same
rate
– Bone marrow cells/skin cells  continuous
division
– Nerve and muscle cells  seldom or never
Cycle Regulators
• The cell cycle is regulated by special proteins called
cyclins and cyclin-dependent kinases.
• High concentrations of cyclin influences a cell to
divide.
• Internal Regulators  proteins that respond to
internal stimuli: cell cycle checkpoints!
– Ex. Cell will not enter mitosis until all chromosomes are
replicated
• External Regulators  proteins that respond to
external stimuli
– Ex. Cell will begin to divide rapidly after injury
– Ex. When dividing cells come in contact with adjacent cells,
division will slow
WHEN CELLS GO BAD!
• Cell loses the ability to
control growth; cancer is the
result.
• Cells do not respond to
chemical signals that tell
them to stop growing.
• Do not differentiate.
• Form masses of abnormal
cells called tumors that
damage surrounding
•May rebuild
tissues.
telomeres and become
“immortal”
Meiosis
• We know that regular somatic (body)
cells contain TWO sets of chromosomes
(diploid/ 2N)
• When a sexually reproducing organism
produces gametes (sex cells) they must
somehow separate these pairs of
chromosomes so gametes only get one
set.
• WHY?
Ex. Humans
• Normal Diploid (2N) somatic cell
contains _____ chromosomes (_____
pairs)
• Gametes (sperm and egg cells) need
to contain _________ chromosomes.
• We generate these HAPLOID (N)
cells through the process of meiosis!
Steps of Meiosis
• Divided into two distinct stages
–Meiosis I
–Meiosis II
• Starts with one diploid cell and ends with
4 haploid daughter cells
• Before meiosis begins, DNA undergoes
replication just like in mitosis!
Meiosis I: Prophase I
• Appearance of the
chromosomes, the
development of the
spindle, and the
breakdown of the
nuclear membrane
(envelope).
• Each replicated
chromosome pairs up
with its corresponding
homologous
chromosome
• Paired chromosomes (4
chromatids) form a
tetrad
Tetrads and crossing over
• It is during this alignment that
chromatid arms may overlap and
temporarily fuse (chiasmata, or
synapsis), resulting in crossovers
• Segments of homologous
chromosomes may switch places
where overlap occurs.
What is Crossing Over?
• Paired-up homologous chromosomes,
may exchange portions of their
chromatids
• Advantage?
Meiosis I: Metaphase I
• Here is where the critical
difference occurs between
Metaphase I in meiosis
and metaphase in mitosis.
In the latter, all the
chromosomes line up on
the metaphase plate in no
particular order. In
Metaphase I, the
chromosome pairs are
aligned on either side of
the metaphase plate.
Meiosis I: Anaphase I
• During Anaphase I
the homologous
pairs separate
from each other
and move along
the spindle fibers
to each pole of the
cell.
End of Meiosis I
• At the end, each daughter cell has a single
set of chromosomes, half the total number
in the original cell where the chromosomes
were present in pairs.
• While the original cell was diploid, the
daughter cells are now haploid. This is
why Meiosis I is often called reduction
division.
Meiosis II
• Meiosis II is quite simple in that it is simply a
mitotic division of each of the haploid cells
produced in Meiosis I.
• There is no Interphase between Meiosis I
and Meiosis II
Meiosis II: Prophase II
• A new set of
spindle fibers
forms and the
chromosomes
begin to move
toward the
equator of the
cell.
Meiosis II: Metaphase II
• All the
chromosomes in
the two cells
align with the
metaphase
plate.
Meiosis II: Anaphase II
• Sister
chromatids
separate as they
are pulled by
spindle fibers
Meiosis II: Telophase II
• A cleavage furrow
develops, followed by
cytokinesis and the
formation of the nuclear
membrane (envelope).
• When Meiosis II is
complete, there will be a
total of four daughter
cells, each with half the
total number of
chromosomes as the
original cell.
Meiosis in Males and Females
• In male animals,
meiosis results in
the formation of 4
___________
cells
• In female animals,
meiosis results in
the formation of
one _______ cell
and three small
polar bodies which
die.
Advantages/Disadvantages of
sexual reproduction?
• Recombination of maternal and paternal
chromosomes in the gamete results in
genetic variation among the offspring. In
an environment which changes, this allows
the process of natural selection to occur.