Transcript Lecture 5

Chromosome, Genome
and Cell Cycle
Dr.Aida Fadhel Biawi
2013
- Chromosomes are used as a way of referring to
the genetic basis of an organism as either diploid or
haploid .Many eukaryotic cells have two sets of the
chromosomes and are called diploid. Other cells that
only contain one set of the chromosomes are called
haploid .
• A diploid cell has two sets of each of its chromosomes
• A human has 46 chromosomes (2n = 46)
• In a cell in which DNA synthesis has occurred all the chromosomes are
duplicated and thus each consists of two identical sister chromatids
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Chromosome Duplication
•
In preparation for cell division, DNA is replicated and the chromosomes
condense
•
Each duplicated chromosome has two sister chromatids, which separate during
cell division
An eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centrometers
Sister
chromatids
Sister chromatids
Chromosome Duplication
• Because of duplication, each condensed chromosome
consists of 2 identical chromatids joined by a
centromere.
• Each duplicated chromosome contains 2 identical DNA
molecules (unless a mutation occurred), one in each
chromatid:
Non-sister
chromatids
Centromere
Duplication
Sister
chromatids
Two unduplicated
chromosomes
Sister
chromatids
Two duplicated chromosomes
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Structure of Chromosomes
• The centromere is a constricted region of the chromosome containing a
specific DNA sequence, to which bind 2 discs of protein called
kinetochores.
• Kinetochores serve as points of attachment for microtubules that move
the chromosomes during cell division..
Metaphase chromosome
Centromere
region of
chromosome
Kinetochore
Kinetochore
microtubules
Sister Chromatids
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Chromosome structure
p: short arm
q: long arm
C: constriction point or centromere , the location of
centromere give the chromosome its shape and can be
used to help describe the location of the genes.
A typical mitotic chromosome at
metaphase
– Diploid - A cell possessing two copies of each chromosome (human
body cells).
• Homologous chromosomes are made up of sister chromatids joined at the
centromere.
– Haploid - A cell possessing a single copy of each chromosome (human
sex cells).
Genome
• The genome is all the DNA in a cell.
– All the DNA on all the chromosomes
• Specifically, it is all the DNA in an organelle.
• Eukaryotes can have 2-3 genomes
– Nuclear genome
– Mitochondrial genome
– Plastid genome
• If not specified, “genome” usually refers to
the nuclear genome.
• A gene is a molecular unit of heredity of a
living organism .It is widely accepted by
the scientific community defined as a
some stretches of deoxyribonucleic acids
DNA that code for a polypeptide or for an
RNA chain that has a function in the
organism.
Human genome
• 22 autosome pairs +
2 sex chromosomes
• 3 billion base pairs in
the haploid genome
Components of the human
Genome
• About 20% codes for proteins
•About 80 percent of the nucleotide
bases in the human genome may be
transcribed. but transcription does not
necessarily imply function , Elizabeth
(2007).
What is the function of the
remaining DNA ?
Much DNA in large genomes is non-coding
• Complex genomes have roughly 10x to 30x more DNA
than is required to encode all the RNAs or proteins in the
organism.
• The non-coding DNA include:
– Intones in genes
– Regulatory elements of genes
– Pseudogenes
– Intergenic sequences
– Interspersed repeats
– Tandem repeats
– Telomeres
• An intron is any nucleotide sequence
within a gene that is removed by RNA
splicing while the final mature RNA
product of a gene is being generated .
• Regulatory sequence is a segment of a
nucleic acid molecule which is capable of
increasing or decreasing the expression of
specific genes within an organism .
Regulation of gene expression is an
essential feature of all living organisms
• Pseudogenes are dysfunctional of genes
that have lost their protein-coding ability or
are otherwise no longer expressed in the
cell .Pseudogenes often result from the
accumulation of multiple mutations within
a gene whose product is not required for
the survival of the organism .
• An Intergenic region )is a stretch of DNA
sequences located between genes . Intergenic
regions are a subset of Noncoding DNA
• .Occasionally some intergenic DNA acts to
control genes nearby, but most of it has no
currently known function.
• It is one of the DNA sequences sometimes
referred to as junk DNA , also known as "dark
matter "
• Interspersed repetitive DNA is found in
all eukaryotic genomes .Certain classes of
these sequences propagate themselves
by RNA mediated transposition, they have
been called retrotransposons ,and they
constitute 25–40% of most mammalian
genomes. Interspersed repetitive DNA
elements allow new genes to evolve .
• Tandem repeats :copies of DNA which lie
adjacent to each other. There are three
types :
1- Satellite DNA - typically found in
centromeres and heterochromatin
2- Minisatellite - repeat units from about 10
to 60 base pairs, found in many places in
the genome, including the centromeres
3- Microsatellite - repeat units of less than
10 base pairs; this includes telomeres ,
which typically have 6 to 8 base pair
repeat units
- Telomeres are regions of repetitive
DNA at the end of a chromosome, which
provide protection from chromosomal
degradation during DNA replication.
Telomeres and Aging:
Is there a connection?
Telomeres are…
- Repetitive DNA sequences at the ends of all
human chromosomes
- They contain thousands of repeats of the
six-nucleotide sequence, TTAGGG
-In humans there are 46 chromosomes and
thus 92 telomeres (one at each end)
What do telomeres do?
- They protect the chromosomes.
- They separate one chromosome from
another in the DNA sequence
(Without telomeres, the ends of the
chromosomes would be fused , leading to
chromosome fusion and massive genomic
instability).
Telomere function
1- Telomeres are also thought to be
the "clock" that regulates how many
times an individual cell can divide.
Telomeric sequences shorten each
time the DNA replicates.
Think of it like this….
2- Telomeres effectively "cap" the end of a
chromosome in a manner similar to the
way the plastic on the ends of our
shoelaces "caps" and protects the
shoelaces from unraveling.
How are telomeres linked to aging?
• Once the telomere shrinks to a certain
level, the cell can no longer divide. Its
metabolism slows down, it ages, and dies.
• It has been proposed that telomere
shortening may be a molecular clock
mechanism that counts the number of
times a cell has divided and when
telomeres are short, cellular senescence
(growth arrest) occurs.
Cell Cycle in Prokaryotic and
Eukaryotic
The Prokaryotic Cell Cycle
1- The prokaryotic cell cycle is a relatively
straightforward process. Essentially, unicellular
prokaryotic organisms grow until reaching a
critical size, and synthesize more cytoplasm,
cell membrane, ribosomes, cell wall, and other
cell constituents. They then replicate their DNA,
segregate copies of the chromosome, and divide
by a process called binary fission to produce two
new genetically identical daughter cells.
Binary fission in prokaryotic
Binary fission in a prokaryotic
1- The bacterium before binary fission is when the DNA
tightly coiled.
2- The DNA of the bacterium has replicated.
3- The DNA is pulled to the separate poles of the bacterium
as it increases size to prepare for splitting.
4- The growth of a new cell wall begins to separate the
bacterium.
5- The new cell wall fully develops, resulting in the
complete split of the bacterium.
6- The new daughter cells have tightly coiled DNA,
ribosomes, and plasmids
2- Most research suggests that the rate of
fission in prokaryotic organisms is largely
controlled by environmental conditions.
For example, most prokaryotic organisms
have an optimum temperature range for
cell growth. When environmental
temperatures are above or below the
optimum, cell division tends to decrease.
3-Under ideal environmental conditions,
many prokaryotic species undergo binary
fission at a fairly rapid rate with generation
times of one to several hours. This can
lead to an astonishing growth in population
size over a relatively short period of time.
In some instances, populations of
prokaryotes may increase by a million or
even a billion fold in a matter of days.
The Eukaryotic Cell Cycle
Phases of the Cell Cycle
• Interphase
– G1 - primary growth
– S - genome replicated
– G2 - secondary growth
• M - mitosis
• C - cytokinesis
Cell cycle begins with the formation of two cells from the division of a
parent cell and ends when the daughter cell does so as well.
Observable under the microscope, M phase consists of two events,
mitosis (division of the nucleus) and cytokinesis (division of the
cytoplasm).
As replication of the DNA occurs during S-phase, when condensation of
the chromatin occurs two copies of each chromosome remain attached
at the centromere to form sister chromatids.
After the nuclear envelope fragments, the microtubules of the mitotic
spindle separate the sister chromatids and move them to opposite ends
of the cell.
Cytokinesis and reformation of the nuclear membranes occur to
complete the cell division.
-Most of the time, cells are in interphase, where growth occurs and
cellular components are made. DNA is manufactured during S phase.
-To prepare the cell for S phase (DNA synthesis), G1 phase occurs (the
preparation of DNA synthesis machinery, production of histones).
-In an analogous manner, the cell prepares for mitosis in the G2 phase
by producing the machinery required for cell division.
-The length of time spent in G1 is variable. In growing mammalian cells
often spend 8-10 hours in G1 phase. G2 is usually shorter than G1 and
is usually 4-6 hours. (brief …..)
Interphase
• G1 - Cells undergo majority of growth
• S - Each chromosome replicates (Synthesizes)
to produce sister chromatids
– Attached at centromere
– Contains attachment site (kinetochore)
• G2 - Chromosomes condense - Assemble
machinery for division such as centrioles
Mitosis
 Some diploid cells divide by mitosis.
 Each new cell receives one copy of every
chromosome that was present in the original cell.
 Produces 2 new cells that are both genetically
identical to the original cell.
DNA duplication
during interphase
Mitosis
Diploid Cell