Chapt 06

GENERAL BIOLOGY
HDL 121
DNA REPLICATION & PROTEIN
SYNTHESIS

PREPARED BY: MANEGA

SCHOOL OF MLT
FACULTY OF HEALTH SCIENCE

DNA REPLICATION & PROTEIN SYNTHESIS
Learning Outcomes

After completing this lecture, students will be able to:
(a) Describe basic chemical substances in cell
(b) Explain physical & chemical characteristic of
the substances
(c) Understand & describe the physiological
functions

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Topics
© 2010 Cosmopoint

Topic Outlines

1.1. Basic Chemical Substances In Cell
1.1.1 Characteristics
1.1.2 Physical & chemical
1.1.3 Physiological functions

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© 2010 Cosmopoint



Nucleic acids

 A macromolecule composed of chains of monomeric nucleotide (linked
together)
 Polynucleotides are formed via polymerisation of nucleotides
 Form polynucleotides by condensation with the formation of phosphodiester
bonds
 Composed of
(a) A base (purine or pirimidine)
Purines: Adenine, guanine which are double ring structures
Pirimidines: Cytosine, thymine & uracil are single ring
structures
(b) A sugar (ribose & deoxyribose)
(c) A phosphate group

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 In biochemistry, these molecules carry
genetic information or form structures
within cells
 The most common nucleic acids are
(a) DNA
(b) RNA (including mRNA, tRNA & rRNA)

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RNA
 Ribonucleic acid
 A close cousin of deoxyribonucleic acid (DNA)
 Polymer of ribonucleoside-phosphates.
 Its backbone is comprised of alternating ribose & phosphate
groups
 Contain ribose while DNA contains deoxyribose

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 Three major types:
(a) messenger RNA (mRNA) carries information about a protein
sequence to the ribosomes, the protein synthesis factories in the
cell
(b) transfer RNA (tRNA) is a small RNA chain that transfers a
specific amino acid to a growing polypeptide chain at the
ribosomal site of protein synthesis during translation
(c) ribosomal RNA (rRNA) & protein combine to form a
nucleoprotein called a ribosome. The ribosome binds mRNA &
carries out protein synthesis

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Functions

 Plays several important roles in the processes of translating
genetic information from deoxyribonucleic acid (DNA) into
proteins
 RNA acts as a messenger between DNA & the protein synthesis
complexes known as ribosomes
 RNA forms vital portions of ribosomes
 Serves as an essential carrier molecule for amino acids to be used
in protein synthesis

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Differences between DNA & RNA
DNA RNA
Double strands of polynucleotide Single strand of polynucleotide
Bigger molecule of more than a million Smaller molecule of less than 1000
bases bases
Entirely double helix strand Single &/or double helix of intra-strand
Pentose within is deoxyribose Pentose within is ribose
Bases consist of A, C, G and T Bases consist of A, C, G and U
The ratio of A + G : C + T = 1 : 1 The ratio of A + G : C + U = 1 : 1
Almost all DNA is found in nucleus Found in both nucleus & cytoplasm
Its amount is constant in all cells, Its amount varies among cells & it can
gametes have half the amount be altered change
Chemically stable Not so stable
Not being broken down, it exists Can be broken down by enzymes
forever
Only one type Three types: rRNA, tRNA & mRNA
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1.1. DNA Replication

Introduction - DNA

 Within the nucleus of every cell are long strings of DNA
 The code that holds all the information needed to make & control
every cell within a living organism.
 Nucleic acid that contains the genetic instructions used in the
development & functioning of all known living organisms.
 Functionally divided into genes.
 Within the molecule, it has specific sequence of bases.
 This sequence of bases codes for a specific sequence of amino
acids in a protein.

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1.1. DNA Replication

 It consists of just a few kinds of atoms: carbon, hydrogen,
oxygen, nitrogen & phosphorus
 Combinations of these atoms form the sugar-phosphate
backbone of the DNA @ the sides of the ladder
 Other combinations of these atoms form the four bases: thymine
(T), adenine (A), cytosine (C) & guanine (G).
 These bases are the rungs of the DNA ladder.
 It takes two bases to form a rung – one for each side of the ladder

DML 202 General Biology & Human
11/16/2011 Genetics (Chapter 11: DNA & 13
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Protein Synthesis)


1.1.1.Process Involved

DNA Replication
 Process in which a molecule of DNA forms 2 molecules
of identical ones that are also identical to the original
one
 Takes place in the nucleus during S
interphase i.e. occurs just before cell division
in both mitosis & meiosis
 Aim: Prepare another identical set of DNA molecules to
make identical sister
chromatids so that chromosome are formed
before cell division can occur

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 Occurs in semi-conservative way as proved by the experiment of
Meselson & Stahl
 An uncoiling enzyme helicase initially catalyses the uncoiling of small
parts of DNA & then proceeds to the whole DNA
 Can proceed simultaneously in
several spots of a DNA molecule

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1.2 Protein Definition

Protein

Large organic compounds made of amino acids
arranged in a linear chain & joined together by peptide
bonds between the carboxyl & amino groups of
adjacent amino acid residues.
The sequence of amino acids in a protein is defined by
a gene & encoded in the genetic code.
Genetic code: a system of representation in which a
code of 3 bases in the DNA or RNA codes for an amino
acid in the protein.

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A primer RNA strand is added to the side of one DNA
strand that acts as a template by the addition of
nucleotide triphosphates forming the new DNA strand
called leading strand
It is catalysed by the enzyme DNA polymerase that
adds nucleotides to the 3’ end of DNA, so replication
proceeds in 5’ to 3’ fashion in the growing new DNA
strand
The bases on these nucleotides are very particular
about what they connect to.
Cytosine (C) will pair to guanine (G); adenine (A) will
pair to thymine (T)

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However, the process is different in the other
complementary strand called lagging strand of DNA
that is bent by the enzyme polymerase & short strands
called Okazaki fragments are formed also in 5’ to 3’ way
DNA ligase: enzyme to join the fragments formed to
become continuous long complementary strand
Finally when the primer is removed by DNA
polymerase, 2 identical strands of DNA are formed
Each contains one side of the original DNA & one side
made of ‘new’ nucleotide

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Protein

Large organic compounds made of amino acids
arranged in a linear chain & joined together by peptide
bonds between the carboxyl & amino groups of
adjacent amino acid residues.
The sequence of amino acids in a protein is defined by
a gene & encoded in the genetic code.
Genetic code: a system of representation in which a
code of 3 bases in the DNA or RNA codes for an amino
acid in the protein.

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1.2.1 Protein Synthesis (transcription and translation)

Can be divided into
Protein Synthesis
(a) transcription
(b) activation of amino acids
(c) translation
(d) formation of functional
protein

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Translation

 The sequence of bases in mRNA is translated to become a
sequence of amino acids in a polypeptide
 Initially, mRNA forms a complex with the subunits of ribosome
 Two sites, P and A are created on the surface of the ribosome with
the first & second codes of the mRNA in them respectively
 The ribosome binds to the mRNA at the start codon (AUG)
 Two suitable amino-acyl tRNAs are then attached to the P site & the
A site respectively each with its anti-codon complementary to that
of the mRNA.

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A peptide bond is formed between the two amino
acids, which are brought close together.
The process is catalysed by a ligase
A relative movement occurs in which the 3rd code of
the mRNA is shifted to the A site & its 1st code is
shifted out liberating the 1st tRNA & the 2nd code sits
on the P site

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Protein Synthesis

Purpose: create a polypeptide – a protein made out of
a chain of amino acids
In a hair follicle cell, a protein called keratin is made.
Many ribosomes can be working on a single strand of
mRNA at once.
Is not a slow process
A protein chain with 400 amino acids long can be
assembled in 20 seconds.

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A peptide bond is formed between the 2nd & 3rd amino
acid lengthening the polypeptide
Another relative movement occurs & the process is
repeated with further lengthening of the polypeptide
until the last code gets into A site

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The last codon is a ‘nonsence’ codon (UAG, UAA &
UGA) indicating a ‘full stop’. No amino acyl-tRNA
can get into the A site. The final relative movement
would liberate the completed polypeptide, mRNA,
tRNA & the subunits of the ribosomes

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DML 202 General Biology & Human
11/16/2011 Genetics (Chapter 11: DNA & 35
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Protein Synthesis)


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Topics

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