CELL AND NUCLEAR DIVISION

            Structure of DNA

- DNA is a poly-nucleotide
- DNA is in form of a double helix in which 2 strands are coiled with each other

                       

Nucleotide: it is made from a combination of deoxyribose sugar (pentose) phosphate molecule and a nitrate base

*In 1 strand many nucleotides are bound with each other through 3-5 phosphate bonds,

*strand opposite to first 1 is called anti parallel since it has 5-3 linkage

*the 2 strands are complementary to each other in such a way that adenine is always with thymine and cytosine is always with guanine

*complementary base attached to each other by hydrogen bonds

*there are 2 bonds between adenine and thymine. 3 between cytosine and guanine

*among the 4 nucleotides 2 are purine ans rest are pyrimidine




Complementary base paring
1- purine and pyrimidine are different sizes
2- adenine forms the same number of H+ bonds with thymine ans uracil
3- the base pairs are of equal length



(DNA polymerase: adds DNA nucleotides to the 3' end of a growing polynucleotide strand )



Semi- conservative replication of DNA: parent DNA is split into 2 strands, each of which is replicated
1- DNA double helix is unwound
2- hydrogen bonds are broken between the complementary base pairs
3- free nucleotides are H+ bonded to those on exposed strand
4- covalent bonds form between adjacent nucleotides on the same strand



Cell cycle:

the stage that a cell passes through after being formed till time it divides

- a large time span between a cell surface is called Interphase in which firstly the cell grows and increases its organelles as well as performs activities increases protein synthesis
- and then, DNA replication takes place
- after DNA replication nucleus division takes either mitosis or meiosis
- last stage is Cytokinesis in which the cytoplasm and the organelles divide into daughter cells

Stages of Mitosis:
Prophase
Metaphase
Anaphase
Telophase



Prophase:
- centrioles divides into a pair and each one moves to the poles and spindle fibres appear from than
- chromatin condenses to form chromosomes
- nucleolus disappears
- nuclear membrane degenerates and disappear too

Metaphase: (chromosomes become shorter and thicker)
- chromosomes arrange on the equator ie, middle line on cell so that the centromers is attached to spindle fibres originate from them
- centromeres splits thus separating the 2 sister chromatids

Anaphase:
- spindle fibres shorten and pull along with the chromatids attached with centromeres towards the pores

Telophase:
- there are now 2 groups of chromosomes on the poles which forms a new nuclear membrane
- nucleolus appear and the nuclear division ends
- after nuclear division, cytoplasm divided into 2 after the organelles are distributed to daughter cells

Mitosis
- a cell division to form daughter cell
- daughter cells are genetically identical
- homologous chromosomes don't pair up in metaphase
- mitosis is the nuclear division needed for growth, tissue repair and asexual reproduction
- reproduction of body parts

Meiosis:
- 4 daughter cell formed
- daughter cell are genetically non-identical
- homologous chromosomes do pair up at equator during meiosis
- reduction division needed for gamete formation for sexual reproduction
- a haploid sex cell which results from meiosis of a diploid cell





During Mitosis:

1- 2 chromatids are joined by a centromere
2- chromosomes line up along the equator of the spindle
3- centromeres divide
4- chromatids move to opposite poles of the cell
5- chromosomes uncoil

DNA mutation:
- a random spontaneous change in DNA is mutation and the agents which increases the chance of mutation are mutagens
for example:
1- ionizing rays such as x-rays, UV rays
2- artificial food colours
3- chemicals, tar
4- viruses

Tumour:
a lump of cells formed due to uncontrolled division and don't specialize. there are 2 types of tumour
1) Benign
2) malignant

Mechanism of Tumour:
- oncogene develop and results in uncontrolled mitosis
- a lump of cells forms a tumour
- this lump is vaginated by blood capillaries and much of the nutrients and oxygen is consumed for their growth and development
- this decreases the efficiency of the organ as it is deprived of oxygen and nutrients
- if some cells of the tumour detach and enter into the blood circulation, this is called Metastasis.
- the treatment of cancer can be chemotherapy, radio therapy or surgical removal of tumor



DNA
* deoxyribose sugar
*  -H
* double strand
* stays in 1 place
* permanent
* reducing sugar
* covalent and hydrogen bonds



RNA:
* ribose sugar
* -OH
* single strand
* can move around
* temporary






Code:
triplet base sequence on DNA sense strand

Codon:
triplet base sequence complimentary to the code, present on messenger RNA (M-RNA)

Anticode:
triplet base sequence present on transfer RNA complementary to codon messenger RNA





Transfer RNA:  (T-RNA)
complementary base paring along some of its length, an area that can attach to a ribosome, a site to which a specific amino acid attaches








> protein synthesis involves 2 stages
> transcription means synthesis of M-RNA according to the code on the sense strand of DNA



Happens as follows:
1) enzyme transcriptase starts transcription and the DNA double helix unwinds
2) the enzyme helicase separates the 2 strands by breaking of hydrogen bonds among complementary base called unzipping
3) result in exposing the base on the sense strand
4) using 2 phosphate each for nucleotides they are activated which move opposite to the respective complementary base on the sense strand



These nucleotides are;

1- Adenine
2- Uracil
3- Guanine
4- Cytosine

and they arrange as A-U and G-C



+ phospho- diester links form between the nucleotides making sugar backbone. this forms a single strand of M-RNA. the enzyme RNA polymerase is used
+M-RNA then leaves the nucleus through nuclear pores and settles on ribosome in the cytoplasm
+ the 2 strands of DNA then coil back




Translation: (linking together of amino acid coded for by M-RNA)
- means synthesis of polypeptide chain according to the codons on messenger RNA
- takes place as follows: (Requires M-RNA, ribosome and T-RNA)
1) M-RNA settles on small subunits of ribosome such that 2 codons are exposed at a time
2) T-RNA with complementary anti-codons bring respective amino acids along so that they arrange on large subunit of ribosome
3) T-RNA leaves after a peptide bond has formed between the 2 amino acids
4) Ribosome then slides a head to repeat the same for next codons until a polypeptide chain is formed
(end product of translation is polypeptide)




Poly- ribosome:
to quicker procedure sometimes 3 ribosome can make a cluster translating 6 codons at a time