1. 9.1 Cell Division and Mitosis
1. Describe the organization of genetic material in a cell.
2. Describe the events that occur during the cell cycle including DNA replication and mitosis
Reading: 5: 109-130 (Reading Quiz due Monday before class)
The purpose of cell division:
Single-celled organisms like bacteria, yeast and some protists reproduce every time a cell divides. In
multicellular organisms cell divisions leads to an increase in size of the organism (growth) or can
replace dead or injured cells (repair).
When a eukaryotic cell, like this amoeba divides all the
organelles are partitioned between the two new cells
(mitochondria, chloroplasts, ribosomes, endoplasmic reticulum,
lysosomes Golgi etc).
Each new cell must also have a complete copy of the genetic
material (DNA).
http://www.microscopy-uk.org.uk/mag/artsep01/amoeba.html
Cell Division has three basic steps:
1. Duplication of the genetic material (DNA)
DNA is decondensed (big ball of string) while it is being copied. It only condenses into chromosomes after it
has been replicated.
2. Movement of the two copies of the DNA to opposite ends of the cell
This is the process of mitosis. Proteins called microtubules attach to chromosomes, pull them apart so that one
copy goes to each end of cell.
3. Cleavage of the cell into two daughter cells
This occurs after mitosis is complete. Cytokinesis = division of cytoplasm
Genome=Sum of all genetic material in a cell, all the DNA,
Our human genome consists of 3.2 Gigabases of DNA carried on 23 pairs of chromosomes
In Eukaryotes, The
DNA from one
chromosome is a very
very long linear
molecule Before cell
division the DNA is
coiled around histone
proteins which allow
it to become folded
up into a much more
compact structure
aka the
chromosome.
Before a cell can divide the DNA must be duplicated! Every chromosome must have two
sister chromatids.
2. 1. DNA Replication
DNA in cells is generally double-stranded
where two chains of nucleic acids are found
zippered together by hydrogen bonds. Each
differvfent nucleic acid is generally
represented by a letter A,C,G,T. There are
specific rules about how nucleic acids on
opposite strands pair. An A on one strand
pairs with a T on the other strand.
C pairs with G.
Being double stranded makes DNA very stable. It helps protect it from damage.
CSI can obtain DNA samples from hair brushes, dried spit, epithelial cells, etc. Even if the cells have died the
DNA is still intact. Intact DNA has been sequenced from some samples up to 100 000 years old….includes
ground sloths, mammoths, neanderthals (FYI: Reports of sequencing DNA from dinosaur fossils or insect
embedded in amber have not been able to be verified by repetition)
DNA replication is “semi-conservative” meaning that the parental strands separate and each one acts
as a template for a new strand. The origin of replication is a place where DNA replication begins
shown as bubbles where the strands open up.
There are many such places along each chromosome
QUESTION: Why is it more efficient to have many origins of
replication along one chromosome?
It’s faster to have multiple replication points than one replication
point.
It takes much less time if many spots are being operated upon.
Replication is directional 5’ 3’
All this means is that the DNA strands have two distinct ends a 5’ end and a 3’ end. When a new
strand is being made it is always made starting at its 5’ end and adding new nucleotides onto its 3’
end. AND it lines up antiparallel to the old strand.
3. QUESTION: If we condensed the DNA before it was replicated we
would have one chromatid we could represent like this
If your new strand was shown in red, after DNA replication is
complete what would the condensed chromosome look like. Where
would the red strands be? Using green and red pens draw your
chromosome with the two sister chromatids.
(Green on outside, green&red mess on the inside). After copying, each
chromatid is made of 1 new strand and 1 Old strand.
Once a chromosome has finished replicating its DNA each chromatid is made up of one new strand
and one old strand. This semi-conservative form of replication is beneficial so that in case a mistake
is made during replication, enzymes in the cell which check the DNA can tell which nucleotide letter is
correct and which one is wrong. Methyl groups are added to the old strand while in interphase.
CH3 CH3
| |
ACGGGCTCAC old strand (template)
TGTCCGAGTG new strand
↑
G and T do not pair. Question: Which one is wrong? (which one will the proof-reading
machinery fix?)
2. Mitosis (division of genetic material)
Cells spend only a very short time dividing. Most of the time cells have
their DNA decondensed inside the nucleus so it just looks like a big ball of
yarn. =INTERPHASE like on the left
QUESTION: When is DNA replicated –when DNA is condensed or
decondensed? WHY?
There are several steps to Mitosis:
1. DNA condenses into visible chromosomes and
the nuclear envelope breaks down.
2. All chromosomes line up at the centre of the
cell.
3. The chromosomes are pulled apart at their
centromeres (so that one chromatid goes to
1 one side of the cell and the other chromatid
3 goes to the other side.)* Both new cells must
have one copy of every chromosome
4. Once all the chromosomes reach opposite
ends of the cell then they decondense again
and the nuclear envelope reforms
2
4
4. This is how chromosomes line
up at the centre so that the
two chromatids go to opposite
sides of the cell.
Your turn:
Using the paper strips we will go through the process of mitosis with a cell that has three
chromosomes.
You should use this space afterwards to sketch out the basic process with a cell with 3 chromosomes.
3. Cytokinesis (division of cytoplasm)
Plant cells have rigid cell wall
Animal cells form a Instead they build a new cell
cleavage furrow wall between the daughter
proteins just under cells with materials brought
the plasma by vesicles from the Golgi.
membrane help pinch = cell plate while it is forming
it in two
CHROMOSOME NUMBER DOESN’T CHANGE IN MITOSIS
Before one of the lily cells divided it had 16 chromosomes. After it divided the two daughter cells
each had 16 chromosomes.
ONLY THE NUMBER OF CHROMATIDS ALTERNATES FROM 1 (after Mitosis)
2 (after DNA replication) 1 (after Mitosis) 2
Further Questions to consider:
5. 1. Give two reasons why DNA is not replicated during mitosis.
2. Below is a chromosome that has replicated its DNA. A few genes designated by letters are shown
on the chromatids. Draw a line through the chromosome to show what is the proper way to pull it
apart so that you get two identical chromosomes.
3. Here is part of a DNA strand of a chromosome in a cell in Interphase. In order for this cell to
divide it must copy the DNA. Label the 5’ and 3’ ends as appropriate. First sketch the strands
separating. Second, show the new strands halfway through copying and indicate which way
replication is occurring. Lastly, show the result after replication is complete.
4. Draw a cell with 4 chromosomes lined up at the centre of the cell. Be sure to draw the
chromosomes in proper orientation so that when the chromosomes pull apart one chromatid goes
to one end of the cell and the other chromatid goes to the other end of the cell for all 3
chromosomes.
