The document describes the structure of chromosomes, the cell cycle, and cell division. Chromosomes are made up of DNA wound around histone proteins. The cell cycle consists of interphase and the M phase. Interphase includes the G1, S, and G2 phases where the cell grows and duplicates its DNA. The M phase involves nuclear and cell division through the processes of prophase, metaphase, anaphase, telophase, and cytokinesis. Mitosis produces two identical daughter cells while meiosis reduces the chromosome number by half to produce gametes.
3. Structure of chromosome
● DNA is wound around a protein known as histone proteins
● When DNA is wound around a histone it is called nucleosome
● They are wound in such a way that the core histones are stacked on top of another
forming a chromatin fibre
● During cell division ,Chromatin fibres further coil to form highly compact well
defined structures called the chromosomes
● Chromosomes are visible only during cell division
4. Structure of the chromosome cont...
● A chromosome has 2 arms (top 1 and bottom 1)
● The place where it comes together is called the
centromere
● There are disc shaped structures on the side of the
chromosomes called kinetochores (helps in cell division)
● A chromosome has 2 sets of identical DNA called sister
chromatids (vertical halves of the chromosomes)
Sister chromatids
Sister chromatids
6. Structure of the DNA (deoxyribonucleic acid)
● The 4 important biological molecules are Carbohydrate, lipids, proteins, nucleic acid
● The smallest unit of a DNA is called nucleotide many nucleotides create a polynucleotide
● Nucleotides are made up of 5 carbon sugar molecule, a phosphate group and a nitrogenous
base
● There are 2 types of nitrogenous bases purines and pyrimidines
● Purines have 2 nitrogen containing rings {adenine and guanine are the purines}
● Pyrimidines have 1 nitrogen containing ring {cytosine and thymine are the pyrimidines}
7. Arrangement of the DNA (deoxyribonucleic acid)
● A DNA molecule is very large and is described as a macromolecule
● Adenine pairs with Thymine with 2 hydrogen bonds
● Guanine pairs with Cytosine with 3 hydrogen bonds
● The 2 strands of DNA are called polynucleotides (look like sides of a
ladder) and are composed of nucleotides (handles of a ladder)
● The DNA is coiled in a double helix strand
8. What are genes
● Genes are specific sequences of nucleotides on a chromosome that encode particular
proteins which express in the form of some particular feature of the body
● They are units of hereditary which are transferred from parents to offsprings and are
responsible for the characters of the offspring
10. Cell cycle
● The first phase of the cell cycle is the mitotic phase
(m-phase)
● It includes splitting of the DNA and
cytokinesis(division of the rest of the cell)
● The phase between 2 m-phases is called interphase
(which is again divided into G1, S, and G2 phases)
11. G1 phase (first growth phase)
● In the G1 phase a new cell is formed which is a result of a recently concluded M-
Phase event
● This cell grows in size (volume of cytoplasm increases)
● Since mitochondria (in animals) and chloroplasts (in plants) have their own DNA
they start dividing
● In the ending of the G1-phase all cells have 2 options
○ They may withdraw from the cell cycle and enter the resting phase (r-phase)
○ Start preparing for the next division by entering the next synthesis phase
12. S - phase (synthesis phase) and G2 - phase (second growth phase)
● Synthesis phase
○ More DNA is synthesized and the chromosomes are duplicated
● Second growth phase
○ This is a shorter growth phase
○ RNA and proteins necessary for cell division continue to be synthesized
○ Now the cell is ready to undergo cell division and enter the dividing phase of the cell cycle
○ After the dividing phase gets completed the newly formed daughter cells enter into
interphase again and the cell cycle goes on
13. prophase
● The chromosomes becomes short ,thick , and visible
● Each chromosome is duplicated to form 2 chromatids
● Centrosomes splits into 2 daughter centrioles and moves
apart
● F
● The 2 daughter centrioles which are called spindle fibres
appear as fibres
● The nuclear membrane and the nucleolus disappears
● The duplicate chromosome (chromatids) start moving
towards the equator
asters
14. metaphase
● Each chromosome gets attached to the spindles by its
centromere
● Chromosomes start lining up in the equator
15. Anaphase
● Centromere attaching the 2
chromatids divides
● The 2 sister chromatids of each
chromosome separate and are
drawn apart towards opposite
poles by means of shortening
of the spindle fibres
● The furrow starts in the cell
membrane in the middle (small
gap)
16. Telophase
● 2 sets of daughter chromosomes reach
opposite poles
● Spindle fibres disappear
● Chromatids thin out in the form of
chromatin fibres
● Nucleoli reappears so Nuclear
membrane and nucleolus reappears
● The cleavage furrow starts deepening
17. Cytokinesis (division of cytoplasm)
● Cleavage furrow deepens completely and split the daughter cells
18. Significance of mitosis
● Maintains the same chromosome number in the daughter cells
● Repair of the damaged and wounded tissues by renewal of lost cells
● Asexual reproduction in which the unicellular organisms such as ameoba or the
yeast cell divide into 2
● Growth of the body size is due to formation of new in the tissues cells
● Repair of damaged and wounded tissues by renewal of the lost cells
19. Meiosis
● It is the cell division which produces the sex cells (gametes)
● It takes place in the reproductive organs
○ It produces sperms and ova in humans
○ It produces pollen grains and female gametophyte in plants
● The most significant aspect in meiosis is that the number of chromosomes in
the sex cells is halved
● One member of each pair of the the chromosomes are passed onto the sex
cells
20. Significance of meiosis
● Chromosome number is halved in the gametes so that on fertilization the
normal number (2n) is restored
● It provides for mixing up of genes which occurs in 2 ways
○ The maternal and paternal chromosomes get mixed up during the first division as they
seperate from the homologous pairs
○ While the maternal and paternal chromosomes are separating the chromatid material very
often gets exchanged between the 2 members of a homologous pair this is known as crossing
over this results in genetic recombination
○ Chiasma is the x shaped structure formed due to crossing over of non sister chromatids of
paired homologous chromosomes
21. Difference between mitosis and meiosis
● In the somatic (body) cells
● Provides for growth and replacement
● Throughout life
● 2 daughter cells
● 1 after chromosome duplication
● Identical
● Full set of chromosomes is passed on to
each daughter cell . this is diploid (2n) no
of chromosomes
● In the reproductive cells
● Only for gamete formation
● Only in reproductively active age
● 4 daughter cells
● 2 after chromosome duplication
● Randomly assorted between gametes
produced
● Only half the number of chromosomes is
passed onto each daughter cell. This is
haploid (n) no.of chromosomes
● Where it occurs
● What for
● When it occurs
● No.of daughter cell
● No.of nuclear division
● Identity of
chromosomes and
daughter cells
● No.of chromosomes in
daughter cells
Mitosis Meiosis
