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 chromosomeschromosomes. . • GeneGene 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

• 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 morphologyi. 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

Property Mitosis Meiosis

DNA replicationOccurs during interphase before mitosis begins

Occurs during interphase before meiosis I begins

Number of divisions

One, include prophase, prometaphase, metaphase, anaphase, and telophase

Two, each including prophase, metaphase, anaphase, and telophase

Synapsis of homologous chromosomes

Does not occur

Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion

Number of daughter cell and genetic composition

Two, each diploid (2n ) and genetically identical to the parent cell

Four, each haploid ( n ) containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other

Role in the animal body

Enable multicellular adult to arise from zygote; produces cells for growth, repair, and in some species, asexual reproduction

Produces gametes; number of chromosome sets by half and introduces genetic variability among the gametes

Summary:

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 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 cyclin-dependent 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”.

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• 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.

• 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.

A molecular control system drives the cell cycle

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.

• 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.

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• 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.

Control of the Cell Cycle

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).

• MPF “maturation-promoting factor” or “M-phase-promoting-factor” 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.

M phase Promoting Factor (MPF)

Rb protein

p53: Tumor SuppressorGene

Cells Which No Longer Respond to Cell-Cycle 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