DPT 1, Lec 7

Cell

Dr Sadia Amer

Learning objectives• To know about the life cycle of cell.• To know about Mitosis and its stages.• To know about DNA replication.• To know about the chemical and

physical events of DNA replication.• To know about the DNA repair.• To know about chromosomes and their

replication.

Life cycle of cellIt is the period from cell reproduction

to the next cell reproduction.• Involves two processes:-MitosisMeiosis

Mitosis• The process by which cell splits into two

cells is called mitosis.Stages of mitosis:-ProphasePrometaphaseMetaphaseAnaphaseTelophase

Definitions• Centrioles:- They are short cylinder like or oval

non membranous organelles located near the nucleus.

They are about 0.4 micrometer long. 0.15 micrometer in diameter. Consists of nine parallel tubular

structures arranged in the form of a cylinder.

• Two pairs of centrioles lie close to each other near one pole of the nucleus .• The two centrioles of each pair lie at

right angles to each other.• These centrioles are also replicated

during interphase

Centromere• Each pair of centioles, along with

attached pericentriolar material is called a centromere.

Chromosome• Chromosomes are made up of joint

molecules of DNA covered with proteins.• The complex of DNA and protein is called

chromatin.• There are 23 homologous pairs in human

somatic cells.• 22 are autosomes• 1 is sex chromosome

Spindle• The complex of microtubules

extending between the two new centriole pairs is called the spindle.• Mitotic apparatus:- The entire set of microtubules

plus the two pairs of centrioles is called mitotic apparatus.

Aster• Before mitosis , the two pairs of centrioles

begin to move apart from each other which is caused by polymerization of protein microtubules.• At the same time other microtubules

grow radially away from each of the centriole pairs, forming a spiny star which is called aster.

MITOSIS

Prophase• Spindle is forming.• Chromosomes of the nucleus

become condensed into well defined chromosomes.

Prometaphase• The growing microtubular spines of aster

fragment the nuclear envelope.• Multiple microtubules from the aster attach to

the chromatids at the centromeres.• The paired chromatids are still bound to each

other.• The tubules then pull one chromatid of each pair

towards one cellular pole and its partner towards the opposite pole.

Metaphase• The two asters of mitotic apparatus are

pushed farther apart.• This occurs b/c the microtubular spines

from the two asters push each other away.• The minute contractile protein molecules

called “motor molecules” composed of muscle protein actin extend between the respective spines.

• As a result of stepping action it slides the spines in a reverse direction along each other.• Chromatids are pulled tightly by their

attached microtubules to the very centre of the cell, lining up to form equatorial plate of the mitotic spindle.

Anaphase• The two chromatids of each

chromosome are pulled apart at the centromere.• All 46 pairs of chromatids are

separated, forming two separate sets of 46 daughter chromosomes.• One set is pulled towards one mitotic

aster and other towards other aster.

Telophase• The two sets of daughter chromosomes

are pushed completely apart.• The mitotic apparatus dissolutes.• New nuclear membrane develops

around each set of chromosomes.• Membrane is formed from portions of ER

that are already present in cytoplasm.

• The cell pinches into two, midway b/w the two nuclei.• This is caused by formation of

contractile ring of microfilaments composed of actin and myosin at the juncture of newly developing cells that pinches them off from each other.

• The stage of mitosis lasts for only about 30 minutes.• More than 95% of life cycle of even

rapidly reproducing cells is represented by the interval b/w mitosis, called interphase.

Replication of DNA• Reproduction takes place in nucleus and first

step is replication of all DNA in the chromosomes.

• Duplication of DNA begins 5 to 10 hrs before mitosis and is completed in 4 to 8 hrs.

• The net result is two exact replicas of all DNA.• These becomes theDNA of the two daughter

cells that will form at mitosis.

• After replication there is another period of 1 to 2 hours before mitosis begins abruptly.• Preliminary changes are beginning to

take place that will lead to mitotic process.

Chemical and physical events of replication

Chemical and physical events of replication

DNA repair• The replacement of defective areas in

DNA and their replacement by appropriate complementary nucleotides is referred to as the DNA proofreading.• This is achieved by DNA polymerases

and DNA ligases which are used in replication.

Mutation• The mistake in transcription process

is termed as mutation.Results in :-Formation of abnormal proteinsAbnormal cellular functionEven cell death

• There are about 30,000 or more genes in human genome.• Period from one human generation

to next is about 30 years.

Chromosome and their replication