2.1.6 Cell Division, Cell Diversity and Cellular Organisation
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2.1.6 Cell Division, Cell Diversity and Cellular Organisation
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Cell Division
6.1 The Cell cycle
6.1 The Cell Cycle
- Cell theory says that all cells come from another existing 6.2 Mitosis
cell. 6.3 Meiosis
- In eukaryotic cells this has two main phases:
o Interphase: 6.4 The organisation and specialisation of cells
• G1 6.5 Stem Cells
• S
• G2
o Mitotic Phase:
• Mitosis
• Cytokinesis
- Cells do not divide all the time; they have phases of growth and ‘rest’. This is called interphase. It is a
very active phase of the cell cycle, with many processes occurring in the nucleus and cytoplasm.
o It is the stage where many metabolic reactions take place.
o DNA replication (in the nucleus) and protein synthesis only happen in this stage.
o Chloroplasts grow and divide in plant and algal cells
o Number of mitochondria in the cytoplasm increases.
- In phase G1 (first growth phase) the cell increases in size, the organelles are replicated, the centrioles
are also replicated.
- In phase S (synthesis phase) DNA is replicated in the nucleus (each of the 46 chromosomes is duplicated
by the cell).
- In phase G2 (second growth phase) cell continues to increase in size, energy stores are increased, and
the duplicated chromosomes are checked for error and any necessary repairs are made.
- In M phase mitosis occurs.
- In Cytokinesis stage the cytoplasm divides and splits into two cells.
- Cells sometimes leave the cell cycle, either temporarily or permanently, this can be for a few reasons
They enter stage G0:
o The cell is specialised to carry out a specific function and is no longer able to divide, it will
function indefinitely and will not enter the cell cycle again.
o The DNA is damaged, it is no longer viable
o Age related diseases such as cancer and arthritis.
- The cell will only move on from one stage of the cell cycle to the next if certain conditions are met.
There are three checkpoints within the cell cycle:
o In G1 to ensure cell size is correct and signals from outside such as growth factors are ok.
o In G2 to check the DNA has replicated correctly and allows for error correction.
o In M phase to ensure chromosomes are attached to the spindle by the centromere properly.
- Cyclins are a group of proteins whose levels rise and fall during difference stages of the cell cycle. They
bind to enzymes called cyclin dependent kinases (CDKs). When they bind to the CDK it is activated.
- Kinases are enzymes that phosphorylate (add phosphate) to other molecules (often enzymes). These
proteins/enzymes will then be activated by cyclins and do tasks associated with a stage in the cell
cycle.
- Mutations in the genes for cyclin proteins can disrupt the regulation of the cell cycle meaning the
checkpoints are not completed and can lead to cancer and tumours.
- Cyclin-dependent kinases can be used as a possible target for chemical inhibitors in the treatment of
cancer. If the activity of CDKs can be reduced it may reduce or stop cell division and therefore cancer
formation.
6.2 Mitosis
, - Mitosis is a type of nuclear division.
- In mitosis both daughter cells are genetically identical.
- Mitosis is necessary when the daughter cells need to be identical such as during growth, repair or
asexual reproduction.
- Before mitosis can occur, all
the DNA is replicated in
interphase. Each chromosomes
are converted into two identical
chromatids; they are joined
together by the centromere.
- During interphase DNA
combines with histone proteins to
form chromatin.
- There are four stages of mitosis:
- Prophase
o The pairs of identical sister chromatids shorten and thicken (condense) as the DNA supercoils.
o The nuclear envelope breaks down.
o The centriole divides and two new daughter centrioles move to the opposite poles of the cell.
o Cytoskeleton protein threads (tubulin) form a spindle between these centrioles.
- Metaphase
o The pairs of sister chromatids orientate themselves at the middle of the cell.
o Using their centromeres each pair attaches to the spindle threads.
- Anaphase
o The centromere divides or splits, separating each pair of chromatids.
o The spindles then contract, whereby the motor proteins walk along the tubulin threads and
pull one sister chromatids from each pair in opposite directions towards opposite poles.
- Telophase
o The chromosomes reach the opposite poles on the spindle.
o The chromosomes uncoil and become long and thin.
o Nuclear envelopes reform around each group of chromosomes.
- Cytokinesis
o In animal cells, a cleavage furrow forms around the middle of the cell. The cell membrane in
pulled inwards by the cytoskeleton until it is close enough to fused around the middle forming
two new cells.
o Plant cells cannot form a cleavage furrow because of the cell wall. Vesicles from the Golgi
apparatus assemble in the middle of the cell, the vesicles fuse together and the cell membrane
dividing the cell into two. New sections of cell wall them form along the new sections of
membrane.
6.3 Meiosis
- Normal cells are diploid (have two chromosomes of each type), one is inherited from each parent.
- In sexual reproduction two sex cells (gametes) fuse to produce a fertilised egg. Gametes must therefore
only contain one of each chromosome.
- Meiosis is the nuclear division that creates haploid gametes, it produces 4 daughter cells. It is known as
reduction division. It also introduces variation.
- Meiosis 1 – the first division where homologous chromosomes are separated into two cells. Each cell will
contain only one full set of genes, so is haploid.
- Meiosis 2 – the second division is similar to mitosis, the pairs of chromatids are separated forming two
more cells, 4 daughter cells are produced in total.
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