Transcript CSE 181 Project guidelines

Chromosomes structure
and Cell cycle
Genetic Material in the Living Cells
o In a non-dividing cell the nucleus is filled
with a thread-like material known as
"chromatin".
o Chromatin is made up of DNA and proteins
(mainly histones and some non-histone acidic
proteins).
o Chromosome is a combination of two
words, i.e., “Chroma”-means ‘colour’ and
“Somes”-means ‘body’.
Chromosomes are the nucleoprotein structures that
carry the genetic information. In eukaryotes they are
located in the cell nucleus.
Chromosomes
• The eukaryotic genome is made up of DNA/protein
complexes called chromosomes.
• Gene sequences embedded within chromosomes
must still be available for transcription by RNA
polymerases and all of the DNA must be capable of
being copied by DNA polymerases.
• Chromosomes have two main functions:
i. To ensure that the DNA is segregated equally to
daughter nuclei at cell division, and to ensure that
the integrity of the genome is maintained and
accurately replicated in each cell cycle.
ii. The elements responsible for these functions are
centromeres, telomeres and replication origins,
respectively
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•
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Each chromosome carries a couple of thousand genes.
Many of these are common to all human beings.
So, 99.9% of your DNA is identical to everyone else's.
The remaining 0.1% influences the differences between us.
 height, hair color and susceptibility to a particular disease.
• Environmental factors, such as lifestyle also influence the way we look and
our susceptibility to disease
Chromosome number/size
•
There are mainly two type:
i. Somatic chromosome number (2n), it is the number of
chromosome found in somatic, merismatic and tissues of species.
ii. Gametic chromosome (n), it is one half of the somatic number
basically it is the haploid number.
•
Chromosome size shows variation depending upon stage of cell
division, it is from 1 to 30 in length and diameter from 0.2
to 2 .
•
Longest and thinnest chromosome seen during interphase.
Chromosome morphology
i. Chromatid: Two chromatids join at the centromere to
form a chromosome.
ii. Centromere: are attachment points for the mitotic
spindle. This attachment is essential for the equal and
orderly distribution of chromosome sets to daughter
cells, thus it is play a key role in stable chromosome
segregation during cell division. Centromere is an A-T
regions.
iii. Telomere: specialized sequences at the ends of a
chromosome that protect and stabilize the entire
chromosome, by protecting the ends from nucleases and
providing unique mechanisms for the faithful replication
of linear DNA molecules. Contain short, repeat TG-rich
element.
Types of chromosomes
There are four types of chromosomes based upon the position of the
centromere.
1) Metacentric: the centromere occurs in the centre and all the four chromatids are of
equal length.
2) Submetacentric: the centromere is a little away from the centre and therefore
chromatids of one side are slightly longer than the other side.
3) Acrocentric: the centromere is located closer to one end of chromatid therefore the
chromatids on opposite side are very long.
4) Telocentric: the centromere is placed at one end of the chromatid and hence only one
arm. Such telocentric chromosomes are not seen in human cells.
Comparison between Mitosis and Meiosis
Summary:
Property
Mitosis
Meiosis
Occurs during interphase before mitosis Occurs during interphase before meiosis I
DNA replication begins
begins
Number of
divisions
One, include prophase, prometaphase, Two, each including prophase, metaphase,
metaphase, anaphase, and telophase
anaphase, and telophase
Number of
daughter cell
and genetic
composition
Occurs during prophase I along with
crossing
over
between
nonsister
Does not occur
chromatids; resulting chiasmata hold pairs
together due to sister chromatid cohesion
Four, each haploid ( n ) containing half as
Two, each diploid (2n ) and genetically many chromosomes as the parent cell;
genetically different from the parent cell
identical to the parent cell
and from each other
Role in the
animal body
Enable multicellular adult to arise from Produces gametes; number of chromosome
zygote; produces cells for growth, repair, sets by half and introduces genetic
and in some species, asexual reproduction variability among the gametes
Synapsis of
homologous
chromosomes
Chromosomes and cell division
• Multicellular organisms copy their
chromosomes before cell division.
• They must grow to a mature size.
• The nucleus divides, distributing the
chromosomes into two equal groups
(mitosis).
• The cytoplasm then divides
(cytokinesis) each part taking a
nucleus.
Interphase
Review Questions
n
n
n
n
n
What are and what happens during the phases of the
cell cycle?
Which proteins are involved in the regulation of the
cell cycle?
Which cyclins and cyclin-dependent kinases are most
important in individual phases of the cell cycle?
What are four mechanisms for regulating cyclindependent kinase activity?
What role do p53, p21, and pRb play in the G1 to S
transition?
The cell cycle
The Cell Cycle
• Cells must accomplish two basic things during the cell cycle:
 Copying cellular components
 Dividing the cell so that components are distributed evenly to
the daughter cells
• The alternating “growth” and “division” activities of the cell is called
the “cell cycle”.
• The “division” activity corresponds to “M phase”.
• The “growth” activity corresponds to “Interphase”.
Continue..
• The timing and rates of cell division in different parts of an animal or
plant are crucial for normal growth, development, and maintenance.
• The frequency of cell division varies with cell type.

Some human cells divide frequently throughout life (skin cells),
others have the ability to divide, but keep it in reserve (liver
cells), and mature nerve and muscle cells do not appear to divide
at all after maturity.
A molecular control system drives the cell cycle
• The cell cycle appears to be driven by specific chemical signals in the
cytoplasm.
• Fusion of an S phase cell and a G1 phase cell induces the G1 nucleus
to start S phase.
• Fusion of a cell in mitosis with one in interphase induces the second
cell to enter mitosis.
Continue...
• The distinct events of the cell cycle are
directed by a distinct cell cycle
control system.
• These molecules trigger and coordinate
key events in the cell cycle.
• The control cycle has a built-in clock,
but it is also regulated by external
adjustments and internal controls.
• A checkpoint in the cell cycle is a
critical control point where stop and
go signals regulate the cycle.
• Three major checkpoints are found in
the G1, G2, and M phases.
• You must know that the order of the cell
cycle is:
Interphase, Prophase, Metaphase, Anaphase,
Telophase, Cytokensis (IPMATC).
• From Prophase to Telophase is what is known
as Mitosis.
Continue...
• For many cells, the G1 checkpoint, the restriction point in
mammalian cells, is the most important.
• If the cell receives a go-ahead signal, it usually completes the cell cycle
and divides.
• If it does not receive a go-ahead signal, the cell exits the cycle and
switches to a nondividing state, the G0 phase.
Most human cells are in this phase.
Control of the Cell Cycle
• The abundance and activity of control molecules pace the cell cycle.
• Some molecules are protein kinases that activate or deactivate other proteins by
phosphorylating them.
• The levels of these kinases are present in constant amounts, but these kinases
require a second protein, a cyclin, to become activated.
• Levels of cyclin proteins fluctuate cyclically.
• The complex of kinases and cyclin forms cyclin-dependent kinases (Cdks).
• Cyclin levels rise sharply throughout interphase, then fall abruptly during mitosis.
Cyclins
• Four classes
Defined by phase of the cell cycle in which they bind their cdk:
• G1/S phase cyclins- bind cdks at the end of G1, commit
cell to DNA replication (cyclin E).
• S phase cyclins- bind cdks during S phase, required to
initiate replication (cyclin A).
• M phase cyclins- bind cdks immediately before
M phase, initiate early mitotic (or meiotic) events
(cyclin B).
• G1 cyclins- involved in progression through the checkpoint
in late G1 (cyclin D).
M phase Promoting Factor (MPF)
• MPF “maturation-promoting factor” or “M-phase-promotingfactor” triggers the cell’s passage past the G2 checkpoint to the M
phase.
i. MPF promotes mitosis by phosphorylating a variety of other
protein kinases.
ii. Stimulates fragmentation of the nuclear envelope.
iii. It also triggers the breakdown of cyclin, MPF only active when
cyclin levels high enough – triggers passage through G2 checkpoint.
• The M phase checkpoint ensures that all the chromosomes are
properly attached to the spindle at the metaphase plate before
anaphase.
• This ensures that daughter cells do not end up with missing or extra
chromosomes.
Rb protein
p53: Tumor Suppressor
Gene
Cells Which No Longer Respond to CellCycle Controls
• Cancer cells do not respond normally to the body's control mechanism.
 They divide excessively and invade other tissues
 If left unchecked, they can kill the organism
• Cancer cells do not exhibit contact inhibition
 If cultured, they continue to grow on top of each other when the total area
of the petri dish has been covered.
 They may produce required external growth factor themselves or possess
abnormal signal transduction sequences which falsely convey growth signals
thereby bypassing normal growth checks
• Cancer cells exhibit irregular growth sequences
 If growth of cancer cells does cease, it does so at random points of the cell
cycle
 Cancer cells can go on dividing indefinitely if they are given a continual
supply of nutrients