Biology Chapter 2.5 Cell Division
3/28/2012 7:06:00 AM
2.5.1 Outline the stages in the cell cycle, including interphase,
(G1, S, G2), mitosis and cytokinesis.
The first stage of cell division is interphase which is divided into 3 phases;
G1, S and G2. The cell cycle starts with G1 (Gap phase 1) during which
the cell grows larger. This is followed by phase S (synthesis) during which
the genome is replicated. Finally, G2 (gap phase 2) is the second growth
phase which separates the newly replicated genome and marks the end of
interphase.
The fourth stage is mitosis which is divided into prophase, metaphase,
anaphase and telophase. During mitosis the spindle fibers attach to the
chromosomes and pull sister chromatids apart. This stage separates the
two daughter genomes. Finally, cytokinesis is the last stage during which
the cytoplasm divides to create two daughter cells. In animal cells the cell
is pinched in two while plant cells form a plate between the dividing cells.
2.5.2 State that tumors (cancers) are the result of uncontrolled
cell division and that these can occur in any organ or tissue.
Tumors are formed when cell division goes wrong and is no longer
controlled. This can happen in any organ or tissue.
2.5.3 State that interphase is an active period in the life of a cell
when many metabolic reactions occur, including protein synthesis,
DNA replication and an increase in the number of mitochondria
and/or chloroplast.
Interphase is an active period in the life of a cell during which many
metabolic reactions occur such as protein synthesis, DNA replication and
an increase in the number of mitochondria and/or chloroplast.
2.5.4 Describe the events that occur in the four phases of mitosis
(prophase, metaphase, anaphase, telophase).
During prophase the spindle microtubules grow and extend from each
pole to the equator. Also chromosomes super coil and become short and
bulky and the nuclear envelope breaks down.
During metaphase the chromatids move to the equator and the spindle
microtubules from each pole attach to each centromere on opposite sides.
During anaphase the spindle microtubules pull the sister chromatids apart
splitting the centromeres. This splits the sister chromatids into
chromosomes. Each identical chromosome is pulled to opposite poles.
During telophase the spindle microtubules break down and the
chromosomes uncoil and so are no longer individually visible. Also the
nuclear membrane reforms. The cell then divides by cytokinesis to form
two daughter cells with identical genetic nuclei.
2.5.5 Explain how mitosis produces two genetically identical
nuclei.
Mitosis is divided into four stages; prophase, metaphase, anaphase and
telophase. During prophase, the chromosomes become visible under a
light microscope as they super coil and therefore they get shorter and
more bulky. The nuclear envelope disintegrates and the spindle
microtubules grow and extend from each pole to the equator. At
metaphase the chromatids move to the equator. The sister chromatids
are two DNA molecules formed by DNA replication and are therefore
identical. These sister chromatids are then separated in anaphase as the
spindle microtubules attaches to centromere and pulls the sister
chromatids to opposite poles. As the sister chromatids separate they are
called chromosomes. This means that each pole has the same
chromosomes (same genetic material). Finally the microtubules break
down, the chromosomes uncoil and the nuclear membrane reforms. The
cell then divides into two daughter cells with genetically identical nuclei.
2.5.6 State that growth, embryonic development, tissue repair
and asexual reproduction involve mitosis.
Growth, embryonic development, tissue repair and asexual reproduction
involve mitosis.
Notes
3/28/2012 7:06:00 AM
Replication: Copying of DNA. Each strand of the double helix separates
from each other and joins with DNA nucleotides to form 4 strands or two
double helix strands of DNA.
Nucleotide: Structural units or molecules of DNA and RNA. Chemicals that
compose DNA eg. Adenine, Guanine, Cytosine, Thymine. Uracil in mRNA
Transcription: DNA becomes mRNA or messenger RNA. Strands split and
rather than pairing with DNA nucleotides, it joins with RNA nucleotides,
and leaves the nucleus.
Translation: mRNA is read by codons of amino acids which form chains
and become proteins for use by cell. Translation occurs outside the
nucleus.
Gene: A length of DNA encoded for a specific purpose.
Chromatin: DNA wrapped around structural proteins of Histone.
Chromosomes: Long chains of DNA wrapped tightly around each other,
super coiled and visible under a light microscope. Exist in two stages.
Before replication they contain one chromatid. After replication they
contain two identical chromatids.
Chromatid: Uncoiled DNA that is too thin to be visible under a light
microscope or strands of DNA within a chromosome. Single piece of DNA
with its supporting proteins.
Histones: Proteins that DNA is wrapped around
Centromere: Point of attachment between two strands of identical DNA.
New Cells are produced by the division of existing cells. The process of a
cell’s synthesis, growth and division is called the cell cycle. The longest
phase in the cell cycle is Interphase. Interphase is a very active period
where a cell grows larger and carries out many biochemical reactions.
DNA molecules within the chromosomes are uncoiled and their genes are
transcribed, allowing for the synthesis or formation of proteins that are
necessary for growth. Mitochondria and/or Chloroplasts increase in
number as a result of increased energy demand by the growing cell.
Interphase consists of three stages:
G1 or Gap One – This is a period of growth, where DNA is transcribed and
proteins synthesized. During G1 each chromosome has only one
chromatid. Cells spend most of their time in G1. It is the time where cells
perform their normal function. Control of cell division occurs in G1. A cell
that isn’t meant to divide remains in G1 whilst those that are meant to
divide enter S Phase after being given a specific signal. Cancer cells enter
the S Phase prior to receiving any signal.
S or Synthesis Phase- All DNA in the nucleus is replicated during this
phase. Chromosomes go from one chromatid to two identical chromatids
that are held together at the centromere.
G2 or Gap Two – The cell now prepares for division. Each chromosome
has two chromatids. During G2, organelles may increase in number, DNA
can begin to condense into chromosomes and microtubules may begin to
form.
DNA is in chromatin form through out this process and is not distinctly
visible as separate units under a light microscope.
Mitosis consists of four stages:
Prophase – Chromatin become super coiled in to chromosomes that are
visible under a light microscope. Chromosomes appear as two identical
sister chromatids joined at the centromere. Centrioles move to opposite
ends or poles of the cell. Spindle Microtubules grow from the centrioles
and extend from the poles towards the equator. Nuclear membrane then
disintegrates and nucleoli disappear allowing microtubules to invade the
nucleus and attach to the centromere of each chromatid.
Metaphase – Double stranded chromosomes move to the centre of the
cell and align themselves along the equator. Chromosomes are still
attached together. Microtubules from both poles are still attached to
opposite sides of the centromeres.
Anaphase – Centromeres divide, separating sister chromatids and
creating separate chromosomes. At this point, each individual
chromosomes goes from having one chromosome with two identical sister
chromatids to two separate chromosomes with one identical chromatid
each. Spindle Microtubules contract and pull the genetically identical
chromosomes to opposite poles.
Telophase – Reverse of prophase. Condensed chromosomes unravel and
nuclear membranes reform around each group of chromosomes. Nucleoli
reappear within the nucleus. The single cell now has two nuclei or two
separate nucleus. Spindle fibres disintegrate. Cytoplasmic division known
as cytokinesis or cell movement takes place, cleaving the cytoplasm in
half and creating two separate yet genetically identical cells.
Cytokinesis: Splitting of animal cell occurs when actin filaments form a
contractile ring around the cell equator which contracts, pinching the cell
in half. Nuclear division of a plant cell involves a new cell wall made of
cellulose forming halfway between the two new nuclei, along the same
axis the chromosomes lined up on during metaphase. Cell membranes
form along the surfaces of this wall. When the new wall joins with the
existing side wall, the cell has become two distinctly separate cells.
Mitosis is used in eukaryotes whenever genetically identical cells are
needed such as during growth, embryonic development, asexual
reproduction and the repair of damaged tissues.
Sometimes the normal control of mitosis in a cell fails, due to a change in
the genes of the cell are a lack of response to a “stop” signal. This cell is a
cancer cell which divides into two cells that inherit the change in the
genes. The two daughter cells divide to form four cells. Cancer cells evade
the immune system and are therefore not destroyed. Repeated
uncontrolled divisions soon produce a mass of cells called a tumour. This
can happen in any tissue and in any organ. Tumours can grow to a large
size and can spread to other parts of the body. The diseases caused by
the growth of tumours are called cancers. A cell that isn’t meant to divide
remains in G1 whilst those that are meant to divide enter S Phase after
being given a specific signal. Cancer cells enter the S Phase prior to
receiving any signal. Normal cells are mortal and can divide about 50
times until losing the ability to do so. This clock gets reset during the
formation of gametes. Cancer cells escape this process and divide
endlessly. Normal cells that suffer significant chromosome damage
destroy themselves due to the action of a gene called p53. Cancer cells
either lose this gene or ignore its message and fail to kill themselves.
Benign cancers are surgically removed and are less serious.
Malignant cancers affect the functioning of organs and often have changes
in the number or structure of the chromosomes in their cells. They also
have altered metabolisms and may spread through the circulatory or
lymphatic systems, spreading to other organs.
There are different types of cancers, however all start with mutations of
specific genes called oncogenes. These genes control mechanisms of the
cell. The mutations in these genes are caused by radiation, carcinogens
and random events during DNA replication. Single cancer cells form
masses of cells called tumours. No further growth can take place uncles
the mass can get its own blood supply. Angiogenesis is the process of
developing a system of small arteries and veins that supply the tumour
with blood. Most tumours do not reach this stage.
A tumour with a blood supply will grow into a large mass. Eventually
some of the cancer cells will break loose and move through the blood
supply to other parts of the body where they begin to multiply. This is
called metastasis and occurs because the tumour cells lose the proteins
on the surface that hold them to other cells, allowing them to break off
and move to other parts of the body.
Cancer can be treated through surgery, radiation or chemicals that kill the
actively dividing cells. It is difficult to remove entire tumours as they
often lack sharp boundaries and metastatic tumours can be very small
and appear anywhere in the body.
Radiation and chemotherapy kill healthy cells as well, meaning treatment
must be balanced to avoid the patients death. Chemotherapy also suffers
from natural selection. Cancer cells eventually develop immunity or
resistance to the chemical, survive the therapy and multiply into a new
tumour. Using multiple drugs can decrease the risk of relapse of cancer as
its hard for a cell to develop resistance to several drugs at the same time.
Mitosis is different to Binary Fission as it is the creation of two daughter
nuclei from one whilst Binary Fission is the creation of two daughter cells
from one cell.