2.5 - Cell Division

Uploaded by luizgmello on 15.10.2012

Hey guys and welcome to topic 2.5, Cell Division. The best way to take in the stages of the
cell cycle is visually. There are great diagrams on Campbell page 232-233. It’s probably
best to watch this video with these diagrams at hand. If you don’t have access to a Campbell
textbook right now, you can find images related to this next to the link to this video on
our course page. -
Interphase is split into 3 stages: G1, where the cell grows in size and matures, S, where
the cell doubles its genetic material and G2, where the cell continues to grow until
it eventually splits into two. Most cells that are not dividing are active but ‘parked’
at the G1 stage. The cycle immediately after G2 is called Mitosis.
This is the actual process of cell division. It contains 4 distinct phases, with some textbooks
distinguishing cytokinesis as a 5th separate stage, rather than a part of telophase.
Interphase and Mitosis together make up the cell cycle. The prophase is the early stage
of mitosis, when the nuclear envelope breaks apart and the centrioles move towards opposite
poles. The mitotic spindle begins to elongate towards the chromosomes.
If you’re following this up with the Campbell diagram, you’ll notice that there’s a
stage there called Prometaphase. The IB actually considers the events of that stage as part
of prophase, so we need to know what happens there, but put it under Prophase.
Metaphase is distinguished visually by the alignment of the chromosomes in the cell equator.
The mitotic spindle produced by the centrioles reaches the centromere of each chromosome.
In Anaphase, the cleavage furrow (possibly the most beautiful word in all of Biology)
begins to form. It basically means the border of the cell is beginning to narrow down the
middle, where the cell will eventually split in half. The chromosome is taken part into
two chromatids and each one is pulled towards one pole of the cell.
In Telophase, the nuclear envelope that was taken apart in prophase begins to reform,
but this time around two separate nuclei in either pole of the cell. The cleavage furrow
is nearly splitting the cell into two separate daughter cells and the mitotic spindle is
being disassembled. Cytokinesis, sometimes included as the last
part of Telophase, is the exact moment where the cleavage furrow snaps the two daughter
cells apart and is by far the shortest of all the stages of mitosis.
- Cancerous cells are directly linked to the
cell cycle. Cancers and tumours are the rapid, indefinite multiplication of cells in a particular
tissue. Healthy cells will spend a genetically-determined about of time in interphase. In cancerous
cells, the mechanisms that regulate the cell cycle become faulty, dictating shorter or
absent periods of interphase. The continuous divisions generate masses of cells called
tumors, which can cause damage to nearby organs or spread to other parts of the body, known
in medicine as metastasis. -
Often times students get the impression that a cell is either dividing (mitosis) or doing
nothing (interphase). That’s absolutely not true! During interphase, the DNA is actively
being read and several parts of it are uncompacted. The reading of DNA means proteins are being
produced. The parts of the DNA not being used are coiled. The chromosomes you see in the
diagrams of mitosis only appear during mitosis because they are super-coiled – extremely
compacted. Mitochondria and chloroplast, both of which
have their own DNA, also divide during interphase as needed by the cell and determined by the
cell function. The replication of DNA, one of life’s most
amazing processes, happens during the S stage of interphase. From all of these, it should
be pretty obvious that the cell is as busy as ever!
- The events of mitosis include the supercoiling
of DNA into chromosomes in prophase, followed by the attachment of the mitotic spindle in
the centromere of the chromosomes in metaphase. In Anaphase, sister chromatids will be taken
apart to opposite ends of the cell and a new nucleus will form around each group of chromatids
during telophase. Cytokinesis takes place when the cleavage furrow permanently separate
the cytoplasm into two daughter cells. (differentiate between a chromatid, sister
chromatids, chromosomes and homologous chromosomes) -
Because the DNA is replicated (copied) during the S-phase of interphase, the two daughter
cells the are the result of the process of mitosis will carry identical genetic material.
- Mitosis serves multiple functions in the body.
Firstly, it’s how multicellular organisms grow in size. Have you ever stopped to consider
that you were once an embryo made of a single cell? How did you arrive at your current 50
trillion cells? One mitotic cycle at a time! One became two, two became four, four became
eight, and so on! So mitosis helps adult organisms grow, as
well as during embryonic development. As the embryo develops, however, not only do the
number of cells increase, they also begin to specialize into functions in separate tissues,
which you saw in topic 2.1 is called ‘cell differentiation’.
Mitosis is also responsible for tissue repair. A good example is the multiplication of muscle
cells to repair the muscle in an injured athlete. Some organisms are also able to reproduce
asexually, in a process called budding. These are natural clones acquired through mitosis.
Sea stars, anemonae, sponges and hydra are example of animals that reproduce this way.