dpt 1, lec 7
TRANSCRIPT
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