10/27/11
Over the years the single-handed non-stop circumnavigation time was
lowered substantially from Knox-Johnston’s 10 months in 1966. In
2003/2004 Francis Joyon lowered it to 73 days on 90ft trimaran IDEC1.
Good question on
DNA as genetic
material
Bad question on the
chemical backbone of
DNA
Easy question about
Nobel Prize for DNA
structure.
Good question about
the 0.34nm repeating
structure of DNA
A
median
B
C
D
F
But how do the A, C, G, and T nucleotide strands fit together?
1
10/27/11
IB104 - Lecture 27 - Mitosis
Reading - Chapter 9
Cells, life cycles, and reproduction
All life consists of cells, so for life to continue, these cells must divide.
This is the cell cycle, and in some cases it is the whole of reproduction,
for example in all prokaryotes and some forms of asexual reproduction in
protists/protozoa. This process of cell division requires that the
instruction set for making cells, i.e. the DNA, be copied accurately into
each daughter cell. Grow, copy DNA, segregate DNA, divide, etc.
In most prokaryotes this is done by
replication of the single circular DNA
chromosome of 1-12 million base pairs,
followed by its segregation to either end
of the dividing cell, and the cell’s other
contents are split too - the process is called
binary fission. Some bacteria can do this
cycle in less than 20 minutes.
Eukaryotic cell cycle
The cell cycle is divided into four phases:
G1 phase for growth of the new cell
S phase for replication (synthesis) of DNA
G2 phase for preparation for division
The actual cell division or mitosis, with multiple stages to it.
Mitosis
G1
S
G2
The mechanical control of cell division as usual is carried out by a
complicated set of proteins, many of which are important in cancer.
There are a series of checkpoints during each cell cycle where the
chromosomes and DNA are checked for damage that must be repaired
before the cell can divide. Cells with excessive un-repairable damage to
their DNA and chromosomes commit suicide, called apoptosis, as you
are perhaps aware if you’ve ever had a bad sunburn and then peeled, as I
routinely did as a child before parents were aware of the need to avoid
such episodes that predispose one to skin cancer later in life.
In eukaryotes, however, there are several complications:
A. There are multiple linear chromosomes, each consisting of an
enormously long DNA molecule. So the genome is typically split among
several chromosomes, ranging from 4 in fruit flies to 23 in us, to over a
hundred in birds and ferns. Recall that the combination of DNA bound as
nucleosomes on histones is called
chromatin because it stains colorfully in
light microscopy with various stains (below).
nucleosome
DNA
The DNA strand is wrapped twice around each core
of histones to make a nucleosome (right), the first
level of compaction of DNA in chromosomes. DNA
histone
core
2
10/27/11
B. Most eukaryotes are diploid, that is they have two copies of each
chromosome, thus every human cell actually has 46 chromosomes. Each
copy is a homolog of the other. Almost all ±30 trillion cells have all 46. 1. Interphase. This period involves
cell growth and replication of the DNA
and is the major time period for
eukaryotic cells (G1, S, and G2).
The chromosomes are replicated, but
remain attached at the centromere,
which is a complex of proteins attached
to the chromosome. Despite its name, the centromere is
not always in the middle of a
chromosome, and in some animals
such as nematodes, there is no discrete
centromere. We now have two sister chromatids
for each chromosome, essentially
identical copies of each original
chromosome, except for rare DNA
polymerase errors.
Interphase
nucleus
C. Division of the chromosomes and their separation into the two
daughter cells requires a complicated process, involving temporary
dismantling of the nucleus, the building of a machinery to separate the
chromosomes, and then division of the cell. In somatic tissues and asexual reproduction this is called mitosis.
In sexual reproduction involving germ cells in the ovaries and testes it is
called meiosis (Monday).
Mitosis
The objective is to copy each chromosome and distribute accurately a
copy of each chromosome to each daughter cell. The numbers of
chromosomes remain the same. Daughter cells are still diploid.
Mitosis is divided into several named stages, during which different
things happen, although of course it is a continuous process in reality.
2. Prophase. This is the first stage in which the chromosomes are visible
as long thin threads in light microscopy. They condense throughout this phase by much folding of the chromatin,
mediated by special histones and many other proteins. The centrioles migrate to take up positions at either side of the nucleus. Recall that this is the second function for centrioles, the other being as
the basal bodies anchoring the microtubules involved in flagella and
cilia.
Early Prophase
pair of centrioles
plasma membrane
Late Prophase
microtubules
centrioles
nuclear envelope
3
10/27/11
3. Metaphase. First the nuclear membrane
disintegrates into tiny vesicles (remember that it
is a double phospholipid bilayer). Then the spindle starts to form with the invasion
of the nuclear region by microtubules from the
centrioles that attach to the centromeres of all the
chromosomes, as well as reaching all the way
through to the other centriole. At the end of metaphase a transitory equilibrium
is reached where all the chromosomes are lined
up on a "plate" in the spindle midway between
the centrioles. The chromosomes are now most condensed, and
it is at this stage that a karyotype can be
photographed through a light microscope. For the
final karyotype photo the individual chromatid
pairs are cut out of the actual photo and rearranged by size (now all done digitally).
Prometaphase
Karyotyping
Metaphase
4. Anaphase. The sister chromatids are pulled/pushed apart from each
other with concomitant separation of the centromere into two parts, and
moved to the opposite poles of the spindle near the centrioles. This involves dissolution of the protein complex holding the centromere
together, and removal of tubulin units from the microtubules at the
centromeres, as well as addition of tubulin units to the microtubules
pushing the centrioles apart.
Anaphase
5. Telophase. This final stage is essentially a reversal of prophase. The
chromosomes de-condense, the spindle collapses, and pieces of nuclear
membrane reform and surround the chromosomes.
Telophase
Interphase
4
10/27/11
Cytokinesis. While the function of mitosis is to divide the chromosomes
precisely to the two daughter cells, the cytoplasm must also be divided,
along with its organelles, etc., but this is done less precisely. It involves
invagination of the cell membrane at approximately the plane of the
metaphase plate, producing a cleavage furrow. Microfilaments of a
special actin protein pull it in like a tightening noose until the cell is
divided into two daughters. Mutants of this actin prevent cytokinesis.
Some of the most beautiful mitosis pictures are from plants. Here the
chromosomes are stained blue while the microtubules are stained red.
But you can also get lovely pictures from animals, like Drosophila flies.
Colors are as in previous slide, but also centrioles are stained yellow.
Cloning of animals This accurate replication and division of the chromosomes each time a
cell divides ensures that every cell has a complete set of genetic
instructions, thus in principle each cell is totipotent, containing all the
information needed to form an animal. In practise this had been difficult
to demonstrate for mammals, although considerable success had been
achieved for other animals like frogs.
In 1996 cloning of a mammal from a single mature somatic cell was
achieved by a Scottish group, using an udder or mammary gland cell
to provide the nucleus to replace the nucleus of an embryo, thus forming
a genetically identical progeny lamb, called Dolly (after Dolly Parton).
This technology is not simple, requiring hundreds of attempts before
success, but has now been replicated in various ways for cows, mice,
horses, pigs, and some other mammals including cats and even recently
monkeys, to the extent that some are concerned about efforts to clone
humans, which are now banned around the world.
5
10/27/11
Cloning is done by nuclear transfer of a somatic nucleus into a donor
egg, which is stimulated to divide into an embryo, which is then
implanted into a surrogate mother for development and birth.
Dolly the sheep
A central concept of biology that also
agrees with the success of cloning of
animals is that the processes and patterns
of development from embryo to adult
involve differential gene expression
regulated by chromatin organization,
DNA methylation at CGs, and all the
CopyCat - the first cloned cat
thousands of transcription factors
mentioned earlier, rather than loss of
genes in cells as tissues develop. The major difficulties with cloning appear
to be that the organization of the histones
and methyl groups on DNA changes in
somatic cells, and this chromatin needs
to be reprogrammed for the genome of a
somatic cell to function properly in a
Tabouli and Baba Ganoush
germ cell and lead to a cloned individual.
(Genetic Savings and Clone)
Dolly Parton
Halloween Dolly
One of the latest developments on this front is that in 2006 (mouse) and
2007 (human), researchers succeeded in “re-programming” adult cells to
be almost like embryonic stem cells, which are obviously capable of
developing into any kind of specialized cell. These are called Induced
Pluripotent Stem Cells or iPSCs, because their status as pluripotent
stem cells capable of differentiation into any kind of specialized cell is
induced by transiently expressing a few crucial transcription factor genes
in them. These transcription factors re-program adult somatic cells into
IPSCs by causing re-modelling of their chromatin. Clinical trials are underway to generate iPSCs from patient’s own skin
cells to replace lost or injured or defective specialized cells like heart
muscle or even spinal cord. While for many this is ethically more
acceptable than using embryonic stem cells (which require destruction of
an embryo for their harvesting), it should also be better immunologically
because they are the patient’s own cells, just reprogrammed. However,
there remains a significant concern about whether some of the iPSCs will
develop into cancers.
6