FUNCTIONS OF CELL DIVISION (LECTURE 20)
What is the purpose of cell division? Reproduction for unicellular
organisms/protist, like budding yeast or amoebas. Dividing from one cell to
two cells is the mechanisms for reproducing the species. Growth and
development: we all started off as fertilized eggs, from on cell we became two
cells, four, eight, etc. As that process occurs, the cells begin to differentiate into
different tissue types. Tissue renewal: the ability of our tissues to regenerate/fix
wounds.
CHROMOSME STRUCTURE & DIVISION
One thing that is going to occur during mitosis is
the copying and replication of the genetic material
and passing on of that genetic material into the two dughter cells. If two cells
are going to be produced, they will have identical genetic info as the parent cell.
The first step in the process of cell division will be copying the genetic matrial
into a chromosome (second step in pic). The original and the copy will be held
together in a region referred to as the centromere. It is a special region of DNA
where two replicate chromosomes are held together. The process of separating
the two sister chromatids is called mitosis. In the context of cell division, the basic function overall is to
ensure that each of the two daughter cells end up with identical genetic info relative to the original
parent cell. The picture is a cell during metaphase where it is the easiest to isolate from cells replicated
chromosomes in a compact condensed state.
HUMAN CHROMOSOME STAINING
This is an example of a karyotype where we have taken cells that were in
metaphase, stained them with a dye that minds to DNA and you end up with
these characteristic binding patterns present on all of the individual
chromosomes.
HUMAN KARYOTYPE ANALYSIS
This is the karyotype of a normal wt. human male with an X and a Y
chromosome and 22 autosomes that are grouped together based on size and the position of the centromere.
When it comes to humans, the normal diploid state is going to be 46 chromosomes, 22 pairs of autosomes and a
pair of sex chromosomes.
FLUORESCENT STAINING TECHNIQUES
By dyeing them, it allows you to compare the individual chromosomes in an individual pair
and make sure that everything is position where it is supposed to be in the context of the
chromosome (the morphology). Because each of these individual dyes are binding
specifically to places within the DNA. For example, if theres an extra copy of a
chromosome, instead of having 46 chromosomes in the karyotype, there would be 47. You
can also get individual changes in chromosome structure, for example, if there was a
difference in the light to dark regions between a pair (the bands), you could suggest that
there may have been a deletion in that chromosome that might cause some genetic
abnormality.
PREPARING FOR CELL DIVISION
When a cell is going to go through the process of cell division, a number
of things are going to have to happen. Initially the cell is going to take
every one of its individual chromosomes, which is conceptually a single
molecule of DNA, and it will replicate it. Before these chromosomes
split, they will be condensed into a M-phase structure. Where the
centromere is formed, we are also going to get a group of proteins
referred to as a kinetochore, which will play an important role in the
separation/segregation of the sister chromatids.
EUKARYOTIC CELL CYCLE
This process of preparing and going through division is a regulated process. It is detailed and there is control
that determines when the cell should be able to actually do this. In the context of eukaryotic cells, that control is
under what we would refer to as the cell cycle. The broadest parts of the cycle would be either interphase or Mphase, where M-phase is the actual process of cell division. Interphase is essentially everything else within the
cell cycle. Interphase can also be subdivided into stages. In the context of interphase, this is when cells are
doing their normal function. In addition to that, part of interphase is going to be preparation for M-phase (cell
division).
Interphase
• normal cell functions; prep for cell division
• G1: Gap 1 or Growth 1 normal cell functions (major regulatory
step when it comes to the cell cycle)
• S (synthesis): DNA replication where the DNA is going to be
replicated. Each individual chromosome is being copied down to
make two copies held together at the centromere that are then going
to enter into M-phase eventually.
• G2: Gap 2 or Growth 2 DNA has just been copied, (remember
cell division is set up to make sure that each of the two daughter cells is an identical copy of the original
parental genome) During G2, there are surveillance/repair mechanisms that are proofreading the copied
DNA to make sure that it was copied correctly, that all of the 3.2 billion base pairs that are present in
your haploid genome were actually copied down exactly as originally designed/present within the
individual nucleus of the cell. Other things: replication of mitochondria (they have their own genomes
which need to be replicated) and mitochondria go through a process that is kind of like binary fission
and the cells will be actively synthesizing biological membranes (new phospholipid bi-layers) so the
smooth ER and all the enzymes involved in phospholipid biosynthesis are absolutely cranking in G2.
We’re about to take one cell and divide it into two cells and that individual pair of cell needs to have
enough membrane to have its own plasma membrane, smooth/rough ER, Golgi, lysosomes, vacuoles,
etc.
Mitotic Phase (occurs in somatic cells of body)
• prophase
• prometaphase
• metaphase
• anaphase
• telophase & cytokinesis
THE EUKARYOTIC CELL CYCLE IS
REGULATED
Cells don’t divide when they want to but are regulated
by checkpoints that the cell has to pass through before continuing on to the next phase. There’s a checkpoint in
G1, at the end of G2, and within M-phase. How are these checkpoints regulated? They are regulated through a
group of enzymes called CDK (cyclin dependent kinases). These enzymes, when activated, are going to allow
the cell through the checkpoints. CDKs are going to phosphorylate target proteins to allow for progression
through the checkpoints by making them active. What regulates the CDKs? A group of proteins referred to as
cyclins. Each one of the checkpoints has their own CDKs with their own specific cyclins. The question being
asked at G1 checkpoint: Does the cell have permission to replicate its DNA and prepare for division? Things
that affect this: nutrient availability, proper contact with neighbors (if a skin cell) this contact activates
proteins that feed into signals that regulate expression of cyclins and activity of CDKs that help the cell
determine whether or not it should divide. If the cell is given permission, then the cell can pass the G1
checkpoint. There is a temporal lag between the G1 checkpoint and S-phase. After the checkpoint, the cell starts
to make a whole bunch of proteins related to cell division, cranks up the synthesis of the DNTPs, the individual
nucleotides that are going to go into all the copied DNA molecules. All of this has to be available for the cell to
do synthesis. Once past, the cell is going to eventually enter synthesis, copy its DNA, go through the DNA
repair, making of membrane, duplicating of mitochondria and chloroplasts of a plant until it reaches the G2
checkpoint. After it has passed G1, proteins will be activated that will degrade the G1 cyclins, which returns the
CDKs to an inactive state.
G2 checkpoint has the DNA all been replicated correctly? Are repair proteins still active?
Do I have enough membrane, mitochondria, chloroplasts? Is the cell ready to enter into
division? Is yes, there is an active mitotic cyclin that will bind to CDK that allows for the
phosphorylation of target proteins to allow the cell to progress through G2 checkpoint and
enter into M-phase.
MITOSIS
G2 of Interphase: all of the DNA in the cell has been replicated. There are two copies of
every single chromosome that is present in the cell and about to enter into M-phase.
Duplicated and uncoiled chromatin combination of DNA (relaxed configuration) and
protein. Nuclear envelope and nucleolus are present. Centrosomes (MTOC) have already
been duplicated, one of the last things to occur in G2.
Prophase: The chromosomes will start to condense, and centrosomes migrate to opposite
sides of the cell. As they begin to migrate, they start polymerizing out
microtubules, forming the spindle apparatus and the asters.
Prometaphase: The initiation of the breakdown of the nuclear envelope,
visiculation. Both centrosomes would have completely migrated to opposite
sides and the asters have completely formed and the spindle apparatus
has
been formed. The spindle apparatus can be separated into two types of microtubules,
nonkinetochore microtubules (overlap with one another) and kinetochore
microtubules (make contact with kinetochore proteins on the centromeres).
Metaphase: Where the M-phase checkpoint occurs do all of the
replicated chromosomes with their kinetochores present, have a
kinetochore microtubule attached to both sides? It’s able to sense this by lining up
the chromosomes along this conceptual line referred to as the metaphase plate. If
each chromosome has kinetochore microtubules attached, then it is ready to progress
to the next phase of mitosis.
Anaphase: The actual splitting of the centromeres and the movement
of the replicated chromatids away from one another (now daughter
chromosomes). Conceptually, half of the chromosomes move towards one pole and the other
half move towards the opposite pole. The cell has now split the replicated genetic material to
opposite poles
Telophase and cytokinesis: T the genetic material starting to relax
(reformation of the nucleolus- where transcription of the ribosomal RNA is
occurring), the nuclear envelope in each of the daughter cells, C the division of the cell
that occurs through the activity of a structure called the cleavage furrow, which forms as the
pinching off between the two cells. What’s occurring is the actin microfilaments that are
tethered to the nuclear envelope are contracting around each other. As they shrink down, because
its tethered to the inside of the plasma membrane, it causes the two cells to pinch apart. During that
pinching apart, cytoplasm, organelles, membranes, and vesicles are split between the two
daughter cells. No longer spindle apparatus but still have some aster formation.
MITOTIC SPINDLE STRUCTURE (Lecture 21)
Aster is collections of microtubules coming out from the centrosomes that are not making contact with the
chromatids/acting themselves as kinetochore microtubules. Instead, they are making contact with proteins
anchored to the membrane. This is important because the cell is starting to elongate (in anaphase). This is the
activity combined of the nonkinetochore microtubules and the asters on the centrosomes. The nonkinetochore
microtubules are overlapping with one another and there are motor proteins that are linking between the two of
them. Motor proteins walk along the microtubules, forcing the microtubules past each other and apart, thus
pushing the centrosomes that have asters, causing the cell to elongate.
ANAPHASE SPINDLE SHORTENING
There are motor proteins associated with the kinetochores, crosslinked to and interacting with the microtubules.
These motor proteins start walking down the length of
microtubule, and in the process of doing so, are dragging the
replicated chromatids apart from one another. As this occurs,
the microtubules are being depolymerized into tubulin subunits.
Kinetochore microtubules in spindle do not “pull” apart sister
chromatids; sister chromatids are “walked” down microtubules
by motor
ANIMAL CELL CLEAVAGE AND CYTOKINESIS
The cleavage furrow is caused by a contractile ring of
microfilaments, which irises closed, pulling the membrane together to
the point that it eventually pinches off into two daughter cells.
MITOSIS IN PLANTS
Stages are basically the same, but there are some differences. Plant cells
are abutted with one another and joined by cell walls; therefore, it
would be hard to move walls apart from each other. Elongation does not occur due to the movement of the
microtubules. The elongation that occurs (only during cytokinesis) is deposition of cell wall in the middle of the
cell to elongate it and then cytokinesis is going to be achieved through the activity/formation of a new cell wall
(a cell plate) between where, ultimately, the two daughter cells are going to form.
PLANT CELL CYTOKINESIS
A cell plate is vesicles of cellulose starting to fuse together. Those cellulose
fibers start to crosslink, and you get a cell plate forming across the middle of
the cell to the point that it eventually reaches opposite sides of the cell and
rejoins both ends to form a new cell wall, thus two new daughter cells.
ASEXUAL v. SEXUAL LIFE CYCLES
Asexual reproduction produces genetic clones of parent (genetically
identical). Sexual reproduction the combination of genetic material (input)
from two different parents to produce an offspring.
CHROMOSONAL STRUCTURES
In sexual reproduction, there will be two copies of all chromosomes. One
copy, conceptually, that came from “mom” and the other from “dad”. Each
of these chromosomes are more or less identical with respect to the genes
that are on them, the genes they encode, and what they do. These two
chromosomes are not 100% identical and there can be subtle changes in the
DNA that allow for different versions of genes. The individual chromosome
that came from “mom” is conceptually the same as the chromosome from
“dad”. Collectively the two of these are referred to as homologous
chromosomes. Organisms that have more than one copy, in this example two
(4 sister chromatids from 2 replicated chromosomes), are referred to as
being diploid.
DIPLOID CHROMOSOME COMPLEMENTS
KARYOTYPE
In a human karyotype the 2n number would equal 46. 22 autosomes in pairs (one from “mom” and one from
“dad”) and then one pair of sex chromosome that determine biological sex.
HUMAN LIFE CYCLE
We all started off as a diploid zygote, a fertilized egg. That cell
then went through successive rounds of mitosis so that you have
a collection of cells in your body that are al genetically identical.
The next generation is going to mean that you need half the
genetic material from mom and half the genetic material from
dad. That process of forming the cells that are called gametes,
sperm cells from haploid and egg cells from haploid females.
The process of making these haploid gametes (each of which
would have 23 chromosomes) is
the process of meiosis.
OVERVIEW OF MEIOSIS
Meiosis is going to start off with a diploid cell with a homologous pair of
chromosomes. These are then replicated to form a homologous pair of replicated
chromosomes. The homologous pairs will come together and during the first
round of meiosis (Meiosis I), we separate the homologous pairs away from one
another. The chromosome inherited from dad will go to one daughter cell while
the chromosome from mom will go to the other. We have cut the number of
chromosomes in half, creating two haploid cells. These cells are technically
haploid but contain replicated chromosomes. The second round of cell division
(Meiosis II) is going to be to separate those sister chromatids away from one
another. There’s going to be another round of cell division that’s going to occur
in the first cell, separate the sister chromatids, and produce two daughter cells
that have one chromatid. The same for the other cell. Collectively, at the end of
meiosis, we go from one parental diploid cell to four individual haploid cells.
MEIOSIS I (LECTURE 22)
Starts off with a cell that is in G2 with centrosomes replicated and begins with
prophase I. The spindle apparatus starts to form, the nuclear envelope starts to
break down, duplicated chromosomes start to condense, and centrosomes migrate
to opposite poles of the cell. However, the difference between mitosis and meiosis
is that homologous pairs of chromosomes are going to come together and align
with one another (called synapsis). The chromosomes literally juxtapose right
next to one another and form a tight complex with a combination of proteins that
hold everything together. During this process of synapsis, crossing over will occur
(genetic material from one homologous arm crosses over with the other arm of the
chromosome). The mechanism of doing this is called homologous recombination. The
genetic material and DNA is similar enough that, as these chromosomes synapse, their
machinery of homologous recombination will form double strand breaks in each of
the individual chromatids and cause them to flip flop. When the chromosomes are synapsed, they from tetrads.
The sites of crossing over are called chiasmas. The proteins that are holding these together are called the
synaptonemal complex (a group of proteins that keep the homologous chromosomes tethered to one another).
A CLOSER LOOK AT CROSSING OVER
Once crossing has occurred, the chromosomes
relax a little bit and we can visualize the
individual chiasmas which represent the points
where crossing over occurred.
MEIOSIS I
During metaphase I, the
kinetochore microtubules
would have made contact with each tetrad, each homologous pair. Notice
that they are still present as tetrads and it is the tetrads that align at the
metaphase plate. The kinetochores microtubules attach to the
kinetochores on each side of the tetrad. In mitosis, it is the centromeres
that align at the metaphase plate. In anaphase I, the pairs of homologous
chromosomes will be segregated from one another.
MEIOSIS II
Telophase I and cytokinesis I: T genetic material can relax and there
may be the formation of a nuclear envelope. C a cleavage furrow will
form to separate the single cell into two daughter cells. There’s going to be a short
gap of time, so if the genetic material did relax and a
nuclear envelope formed, so that all of that can be
undone when we enter into the next step of meiosis.
Biggest thing that will occur before the start of
Meiosis II will be the replication of the centrosomes.
Prophase II what happens depends on what
occurred in TI, if chromatin relaxed, it will condense
again. If nuclear envelope reformed, it has to rebreak
apart. The centrosomes are going to form a spindle
apparatus and migrate to opposite sides. Metaphase
II All of the replicated chromosomes will be
captured by
kinetochore molecules and lined up at the metaphase
plate. In this step, the centromeres are going to be aligning along the equatorial plate in each
daughter cell. Anaphase II the centromeres/sister chromatids will split and move towards
opposite moles of the cell. Telophase II and cytokinesis II: T in most instances, genetic
material will relax and there will be some formation of a nuclear envelope. Ccleavage furrow
in both daughter cells and separation into four separate haploid cells.
MITOSIS V MEIOSIS
SEXUAL LIFE CYCLES
Around 44 mins.
CHROMOSOMES ASSORT INDEPENDENTLY
Stopped at 53:21
CROSSING OVER ADDS GENETIC VARIATION
SOURCES OF GENETIC VARIABILITY
• independent assortment of chromosomes
– random orientation of homologous pairs at metaphase plate during metaphase I
– 2n for # of possible combinations; 223>8 million!
• random fertilization
– any sperm with any egg!
– 223 x 223 > 70 trillion possibilities
• crossing over during prophase I
– combines DNA inherited from two parents into a single, heritable chromosome