BIOL 112 Lecture Notes Part 2.doc

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17 Mar 2012
BIOL 112/Winter 2011/Lecture Notes Part II
BIOL 112 (Winter 2011) notes for Part II: Information and Heredity (Professor Dent's section). From February
16, 2011 to whenever the semester ends.
Prof does not have office hours - email him to schedule an appointment, or just email him
3 sets of practice problems - do them
A nice story about a central Asian monk named Temujin around 1220 AD and unknown paternity
(aka Genghis Khan)
Had to give his land to his son, but paternity issues, some sort of controversy
Sort of illustrates the concept of heredity, which is what we're learning now
The mystery of heredity
All organisms come from other organisms (except for the base case, which ostensibly was
All organisms resemble their parents
Siblings are not identical
The magic that is conception
An egg becomes fertilised and undergoes cleavage (cell division) until eventually you get an
Cell cycle
All organisms consist of cells, which divide to produce new cells
Higher organisms fuse their cells (sperm and ova) to produce a new organism
Cell division results in more cells, etc
Dividing cells devote a lot of resources to segregating chromosomes
i.e. organising them in a line, pulling them apart so that each cell gets an equal number of
Chromosome: single string of DNA
Circular: bacteria
Linear: most other organisms (including us)
When a cell is ready to divide, chromosomes condense, associate with proteins (e.g.
Combination of DNA + proteins = chromatin which is dark and easy to see
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Normal DNA is difficult to see under a light microscope, but when it's ready to divide, easier to
The DNA double helix wraps around histones, which wraps around other proteins, etc ... all
bunched together
Karyotype: organising and identifying chromosomes
Take a cell that is getting ready to divide
Flatten it with glass (mitotic squash?)
Stain it with a dye, gives them stripes; take a picture and cut them up etc
Shows that chromosomes come in pairs - homologs (except for the sex chromosomes, which
don't look like each other)
The number of chromosomes is characteristic of a specie (with some exceptions)
Humans typically have 46, roundworms have 2, pigeons 80
Doesn't really correspond to complexity or anything
Just before cell division, each chromosome has been replicated once to produce two chromatids (at
the top of the chromosome)
Two chromatids for each chromosome in a pair, held together by a centromere
Normally a chromosome is a single piece of DNA
But when we look at a karyotype, we are seeing two pieces of DNA - the two chromatids,
bound by a centromere
We call these mitotic chromosomes to show that they are a special case
Segregating chromosomes is exacting: a cell needs to give each daughter cell the right number of
With random segregation, only half the time does each daughter get one chromatid (the right
So there needs to be some sort of mechanism that ensures that the right result is enforced
every time
Organisms need at least one of each chromosome, and typically exactly one
more can cause problems - ex, Down syndrome - an extra chromosome 21 (which is
incidentally the smallest chromosome)
So the chromosomes must obviously be duplicated BEFORE the cell divides
NOW THE ACTUAL CELL CYCLE (steps of cell division)
Chromosome (DNA) replication: S phase
Mitosis (M) somatic cells divide into daughter cells, each of which inherits one copy of each
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OR Meiosis (M) for germ lines (gametes) - non-identical copies, creating daughter cells that
have one of each homolog
Cytokinesis: dividing the cytoplasm in two (optional)
Starts with Gap 1 (G1) - cell is growing, accumulating resources it needs to divide
Then it duplicates its chromosomes - DNA synthesis (S phase)
Gap 2 (G2) - another rest phase, preparing to divide
Then it undergoes mitosis (M), and the daughter cells go through the cell cycle again
To ensure that this occurs in order, there is a checkpoint between each phase
From S to G2, cell checks that each chromatid has been duplicated
What evidence of this checkpoint do we have?
Drug: hydroxyurea, blocks chrosome replication
So cell division is put on hiatus indefinitely
Another: caffeine, which disables the checkpoint
This is usually not a problem, as chromosome replication occurs without problems
However, if you both block chromosome replication AND disable the checkpoint, you
have problems
Because the cell will attempt to continue to the mitosis stage, but will find that it's not
able to divide properly
Cell cycle tightly regulated to ensure that, in particular, each cell gets the right number of
Diagram later maybe
More on the G1 to S transition:
There is a protein called Cdk4, which is always present during the cell cycle but doesn't really
do anything
But then it associates with another protein called Cyclin D, which is only produced during S
This complex interacts with other proteins to say that the cell is ready to go into S
Cyclin is degraded, Cdk4 is released (no longer functional)
So this activity of proteins tells the cell what cycle it's in, when to start transition
Although cyclins and cdks are involved in all parts of the cell cycle, important in different
transitions, etc
How do cells know when to divide?
Most somatic cells not dividing - arrested in the G1 phase of the cycle
Often waiting for signals from other cells to tell them to divide
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