6. DNA
• DNA Structure was discovered by James
watson and Francis Crick in 1953 for which
they were awarded with Nobel prize.
• However their work was based on Rosalind
Franklin.
• The double helical strand of DNA is also called
the Watson and Crick structure.
14. DIFFERENCE BETWEEN DNA AND RNA
DNA RNA
•Stand for Deoxyribonucleic
acid
Stands for Ribonucleic acid
•It has a double helical strand. It has a single helical strand
•Contains the base
Adenine,Thymine,Guanine,Cyt
osine
Contains the base
Adenine,Uracil,Guanine,Cytosi
ne
•Present in Nucleus of the cell Present in both nucleus and
cytoplasm
•Sugar present is deoxyribose Sugar present is Ribose
15. TYPES OF NITROGENOUS BASES
• There are two general types of nitrogenous bases in both DNA and RNA-
Purine and Pyrimidine.
• Two common purine base found in DNA and RNA are Adenine and
Guanine.
• Two major pyrimidine bases found in DNA are Thymine and Cytosine and
in RNA they are Uracil.
» Adenine(A)
» Thymine(T) in case of DNA /Uracil(U) in case of RNA
» Guanine(G)
» Cytosine(C)
Adenine base pairs with Thymine
Guanine Base pairs with Cytosine
16. PURINE AND PYRIMIDINE STRUCTURE
•Purine is a heterocyclic aromatic organic compound consisting of a pyrimidine
ring fused with imidazole ring.
•Pyrimidine is a heterocyclic organic compound similar to benzene containing
two nitrogen atoms at position 1 and 3 of the six member ring.
Fig- Purine Fig- Pyrimidine
18. BASE PAIRING RULE
• Adenine only base pairs with Thymine.
• Guanine only base pairs with Cytosine.
• A and T are complementary bases
• Similary G and C are complementary bases.
• Complementary bases always link with each
other and never with other bases. This is known
as base pairing rule.
• The amount of adenine and thymine are always
equal in each molecule of DNA. So Is the case
with Guanine and cytosine.
19. REPLICATION OF DNA
• Replication of DNA – an exact copy of each
DNA molecule is made in the nucleus.
• As a result of replication each cell receives an
the same amount of DNA.
• Replication of DNA occurs in the S phase of
Cell cycle
21. REPLICATION OF DNA
• First the Unwinding of DNA occurs by the
enzyme Helicase.
• DNA synthesis then occurs by another enzyme
DNA polymerase.
• DNA synthesis always proceeds in 5’ to 3’
direction.
• Each strand acts as a template for the
formation of new strand of DNA.
23. GENETIC CODE
• DNA contains the information for the
manufacture of protein. Out of a long stretch of
DNA Only a certain segment of DNA code for a
particular protein. The segment of DNA that
codes for a protein is called Gene.
• Only one of the strand of DNA actually codes for
the manufacture of protein in a cell. The strand
that codes for this information is called template
strand.
• The other strand is called non template strand or
coding strand.
24. GENETIC CODE
• Proteins are made up of a chain of amino acids.
• A sequence of three bases codes for one amino acid.
• Because three bases are required to code for one amino
acid, the DNA code is a triplet code.
• The first codon is called the start codon.
• The last codon is called stop codon.
• Base sequence that code for a particular amino acid may
vary.
• The triplet code that is same in all the organisms is called
Universal code.
• For eg- TGT codes for the aa Cysteine in
humans,bacteria,banana,or in any other organism.
25. PROTEIN SYNTHESIS
• Takes place in two stages
» Transcription- Synthesis of mRNA from DNA
» Translation- Conversion of the mRNA code into
a protein
• Transcription occurs in Nucleus
• Translation occurs in Cytoplasm.
• Two types of RNA takes part in protein synthesis
» Messenger RNA(mRNA)
» Transfer RNA(tRNA)
26. TRANSCRIPTION
• DNA helix is unwinded.
• The template strand is then used to build
mRNA from DNA by adding RNA nucleotides.
They are added one at a time.
• Transcription is performed by enzyme RNA
Polymerase
• They too follow the complementary base
pairing rule.
31. TRANSLATION
• In the cytoplasm the code in mRNA is converted into a
protein.
• Protein synthesis takes place in Ribosomes.
• The triplets of bases in mRNA is called codon.
• The codon AUG AND CUG are the initiation codon that
codes for the amino acid methionine.
• The mRNA attaches to the ribosome.
• tRNA has anticodon arm that is complementary to the
bases of mRNA
• Other end of tRNA is a site where a specific amino acid
can attach.
33. TRANSLATION
• tRNA First binds to mrna at the start codon which
is always AUG/GUG. This codon codes for amino
acid Methionine.
• Another tRNA brings along a second amino acid.
The anticodon of second tRNA binds to the next
codon on mRNA.
• A bond forms between the methionine and
second amino acid.
• The first tRNA is released and goes off to collect
another amino acid.
34. TRANSLATION
• More tRNA molecules arrive at the mRNA and
add their amino acid to the growing chain
forming a protein.
• At the end of the chain a stop codon tells the
translation machinery that the protein is
complete and it is released.
36. AMINO ACIDS
*Note-It may be noted that the amino acids selenocysteine and pyrrolysine are
considered the 21st and 22nd amino acids, respectively. They are more recently
discovered amino acids that may become incorporated into protein chains during
ribosomal protein synthesis.
