the helical structure of DNA is explained and the invovlement of DNA in protein synthesis is explained for nursing students.

1. STRUCTURE OF DNA
MRS.TINAANN JOHN
ASSOC. PROFESSOR
CHILD HEALTH NURSING DEPARTMENT
HOSKOTE MISSION INSTITUTEOF NURSING

2. WATSON CRICK MODEL OF DNA
• In the year 1953Watson and Crick proposed a model for
DNA structure that is known as the “ Double Helix Model”
• They were awarded the Nobel Prize in 1962 for these
contributions to genetics.

3. DOUBLE HELIX MODEL OF DNA
• It consist of:
1. Two right handed helical polynucleotide chains
2. These 2 chains lie side by side but run in opposite directions
or opposite polarity(anti –parallel).
3. The two chains are plectonemically coiled around the same
axis to form a double helix.
4. The bases are on the inside of the helix in pairs arranged in
such a fashion that a pyrimidine of one chain always pairs
with a purine of the opposite strand and vice- versa.

4. Contd…
• The two chains are held together by hydrogen bonds the
nitrogenous bases.
• According to Chargaff’s rule Adenine(A) always pairs with
Thymine(T) and Cystine (C)
• With Guanine(G).
• A=T C=G
• The pairing rules require that the bases in the two chains are
complimentary so that the sequence of one chain is completely
determined by the sequence of its partner

5. Contd..
• The distance between the two successive nucleotide is 3.4 A
• Each turn of the double helix covers a distance of 3.4 A and
involves 10 pairs of nucleotides.
• Each nucleotide is turned 36 from the preceding one so that a
complete 360 involves 10 base pairs.
• The diameter of the helix is approx 20-22 A and its possible
only when there is a pairing of purine with a pyrimidine.
• The length of the DNA filament of a single chromosome is 50
mm (Chromosome length -5µ)

7. RIGHT HANDED DNA HELIX
• It consists of 2 strands of nucleotides twisted around each
other with the backbone of the chains tracing a smooth
right handed turn.
• B- DNA the double helix described byWatson and Crick
present in all DNA or living things.
• A- DNA and C-DNA Dehydrated DNA present in certain
solvents.

8. LEFT HANDED DNA HELIX
• It is discovered by Rich of Massachusetts Institute of
Technology which is called Z-DNA.The back bone of the
strands follow a zig zag course.
• Rich et. al. suggested that when the genes are stabilized
by methylation, a regulatory protein binds to the sites
and keeps the gene turned off and forms z-configuration.
• Demethylation causes the gene to turn on when the
regulatory protein lets go.

9. DENATURATION AND
RENATURATION
• G=C rich DNA molecules are comparatively more resistant to
heat than A=T molecules.
• The H bonds break if DNA molecules are heated to nearly 100
C and the two strands separate.This is called denaturation of
DNA.
• If the heated DNA solution is slowly cooled the two strands
pair again.
• The process of denaturation and renaturation can be used in
synthesizing hybrid DNA molecules.

10. RIBONUCLEIC ACID-RNA
• RNA is present in the nucleolus and cytoplasm.
• In human beings RNA is not concerned with inheritance.
• It is synthesized from DNA template.

12. RNA STRUCTURE
1. Building blocks of RNA
a. The basic components of RNA are the same as DNA with two
major differences.The Pyrimidine base uracil replaces thymine
and ribose replaces deoxyribose. (A=U)
2. Nucleosides and Nucleotides
a. Nucleosides are formed similarly to DNA with ribose replacing
deoxyribose uracil.

14. TYPES OF RNA
• Messenger RNA (mRNA)
• Ribosomal RNA (rRNA)
• Transfer RNA (tRNA)

15. MESSENGER RNA
• It is synthesized inside the nucleus.
• It is formed as a complimentary chain in the DNA strand by the
process of transcription. It carries all the genetic information in
the form of sequence of base arrangement however the
thymine of DNA is replaced by Uracil of mRNA.
• mRNA consist of several of thousands of nucleotides arranges
in a single strand.
• After formation of mRNA in the nucleus it comes out through
the nuclear pores into the cytoplasm and attaches to the
ribosomes.

16. mRNA Contd….
• The life span of the mRNA varies from few hours to few
days.
• The mRNA constitutes 10% of the total RNA.

17. RIBOSOMAL RNA
• The rRNA occurs in the ribosome and constitute about 80% of
the total RNA present in the cell.
• It is synthesized inside the nucleus.
• The part of the DNA which codes for rRNA is associated with
nucleolus and is called nucleolar organizer.
• The ribosome is made up of two subunits that is large and
small.
• The rRNA occurs in the form in the form of 28s, 18s, and 5s
dimensions.
• rRNA helps in the synthesis of proteins.

18. TRANSFER RNA
• tRNA molecules is a single stranded and consists of about 75- 80
nucleotides.
• It is synthesized at particular region of DNA.
• The message derived from DNA is mRNA but this message cannot be
directly read by amino acid which is done by tRNA.
• The tRNA transfers amino acid alkaline on to the ribosomal surface
at the time of protein synthesis and is named as alanine tRNA.
• The alanine tRNA consists of 77 nucleotides and has a high content of
unusual bases which expose the free keto and amino groups to form
secondary bonds with mRNA for protein synthesis.

19. tRNA Contd…
• The molecule of alanine tRNA has the following special
sites:
• AminoAcid Attachment Site: the 3’ end consist of ACC (Amino
Acid Acceptor) which is the terminal nucleotide Adenosine (A).
The 5’ end terminates in the phosphate and Guanidine (5
phosphate (G))
• Ribosomal Recognition Site (first loop): it is away from the 3’ end
consists of several unpaired bases (UGCAUU-3).This region is
involved in the binding of the ribosomal surface.

20. tRNA Contd..
• Anticodin Site (Third Loop): the second of the central loop
consists of seven unpaired bases.Tis loop contains the anticodin
at the bases with purine at 3’ end and uridine at 5’ end.
• AminoAcid Recognition Side (Fourth Loop): the fourth loop is
the binding site of the specific activating enzymes (amino acyl
synthetase) and consists of 8-12 unpaired bases relatively in
dihydro-uridine.

24. REPLICATION AND
SYNTHESIS OF NUCLEIC ACID

25. REPLICATION AND SYNTHESIS OF
NUCLEIC ACID
• Nucleic Acid is the genetic material and it must be
capable of being replicated so that it can transmit in the
progeny.
• The chromosomes duplicate themselves during cell
division at the end of mitosis.
• The duplication of chromosomes means the duplication
of DNA.

26. REPLICATION PROCESS OF DNA
• The double helix model of DNA undergoes simple mechanism of self
replication.
• The steps are as follows:
• Untwisting of the DNA molecule and separation of the two strands
• During the 5th phase of cell division the tightly coiled DNA filament gets
uncoiled.
• The DNA molecule untwists to separate the 2 strands
• The H- bonds break and the 2 strands break and the 2 polynucleotide chains
thud separated are complimentary to each other.
• This unwinding occurs with the help of many proteins and actions of enzymes.

27. Process Contd….
• Formation of complimentary strands on each free single
strands
• Each single strand serves as a primary DNA molecule and is the
direct template for its own formation.
• Each purine or pyrimidine base attracts a complimentary free
nucleotide available in the cytoplasm of the cell.
• The newly attached nucleotide are held in position by the H Bonds
between them and the template bases.
• The sugar radicals of neighbouring nucleotides are then attached to
each other through their phosphate component which is aided by
DNA polymerase enzyme.
• Thus a new nucleotide chain is formed opposite to the old
polynucleotide chain.

28. Process Contd….
• Formation of two daughter molecules each with one
parental and one new strand.
• After the growth of the complimentary chains along the full length
of both parental polynucleotide strands 2 DNA molecules are
generated with identical sequences of the bases thus carrying the
same information content as the parental DNA molecule.
• In the next replication cycle the 4 polynucleotide strands again
separate each acting as a template for the synthesis of its
complimentary strand.
• The method of DNA replication is described as semi conservative as
only one strand of DNA is newly synthesized while the other strand
is of the parents.
• After the 2nd replication cycle two of the four grand daughters
double helices contains one parental and one new strand where as
the remining two grand daughters double helices contain both new
and non-parental strands.

31. PROTEIN SYNTHESIS

32. PROTEIN SYNTHESIS
• Proteins can function as:
• Enzymes
• Repairing agents for the cell
• Proteins determine the shape and function of the
organism.
• Each gene is responsible for one protein.The message for
the DNA code is information for building proteins.

33. • GENES CODE FORTHE PROTEINS BY USING A GENETIC
CODE
• The nitrogen bases set up as set of 3 that are known as
CODONS.
• More than 1 codon can code for an amino acid, however
for any one codon there can be only one amino acid.
• Genetic code is universal.The codons represents the
same amino acids in all organisms.

34. STEPS OF PROTEIN SYNTHESIS
• The three basic steps of Protein Synthesis is:
1. Transcription
2. Transferal
3. Translation

35. TRANSCRIPTION
• This is the first step in the formation of protein.
• It is the process of transmission of the genetic information
stored in the DNA strand to the messenger RNA (mRNA)
• In the nucleus the enzymes make an RNA copy using a portion
of the DNA which becomes the mRNA which leaves the
nucleus through the nuclear pores into the cytoplasm of the
cell.
• The mRNA mixes with the ribosomes which begins the process
of protein synthesis.

36. Contd…
• The DNA strands separate from each other and the
transcription starts from the 5’ end and ends at 3’ end.
• Each base formed in the newly synthesized mRNA is
complimentary to a corresponding base in the DNA.
• In the mRNA all the sequence of a structured gene are
transcribed that is exons and introns.
• The intervening non-coding sequences (introns) are excised.
The exons are spliced together to form a mature RNA which is
shorter in length.
• Splicing is the process of removal of introns by cutting them
off and joining the ends of extrons.

37. Contd…
• The mRNA strand has the initiation and termination
codons.The initiation codonAUG is present on the 5’end
which marks the beginning of the message for a
polypeptide chain.
• The termination codon UAA or UAG is present at the
3’end which terminates the formation of the polypeptide
chains.

39. TRANSFERAL
• In this stage an ATP molecule attaches to the amino acid .
• The ATP and the amino acid attaches to each other and
releases 2 phosphates. At this point the amino acid is
energized.
• The adaptor (amino acid activating enzyme) then attaches
itself to the amino acid.
• The adaptor (AAA) comes closer to the amino acid until they
bond.
• Energy from the ATP molecule is transferred into this bond and
the ATP molecule is released.
• The activating enzyme finally releases the adaptor with the
amino acid attached to one end.

40. TRANSLATION
• At this point, one amino acid is attached to an AAA enzyme.
• It takes many amino acids to make up one protein.Therefore
there must be a way to link these amino acids into a single
protein in order to complete the protein synthesis.
• This where the ribosomes reads the start codon (AUG) and
associates it with the proper amino acid.
• Once all the amino acids have been linked the mRNA stops.
• The ribosomes release the mRNA and the amino acid and a
protein is made.

41. • TheTranslation process occurs in 3 steps.
• Initiation: During the initiation phase an ribosome
attaches to the mRNA and reads the codon (AUG)
• Elongation: During this phase , the tRNA brings the
corresponding amino acids to each codon, as the
ribosomes moves down the mRNA strand.
• Termination: During the last phase, the molecule is read
and the synthesis ends and releases the protein.