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Chapter 6
Nature and Action of the Gene
Structure of DNA molecule
The basic unit or
monomer to build
up a DNA molecule
is called
deoxyribonucleotid
e or nucleotide. It
consists of three
parts:
P
CH2
O
Organic base
H
H
1) Deoxyribose
2) Phosphate group
3) Organic base or
nitrogenous base
OH
OH
Structure of DNA molecule
Nitrogenous bases:
1. Purine
a. adenine
b. guanine
2. Pyrimidine
a. thymine
b. cytosine
Structure of DNA molecule
The deoxyribonucleotides are linked by phosphodiester
bond to form a polydeoxyribonucleotide chain (or
polynucleotide chain).
The phosphates and
deoxyribose form the
backbone structure of the
polynucleotide chain while
the nitrogenous bases are the
side chains.
Structure of DNA molecule
The two polynucleotide
chains join together by
the hydrogen bonds
between complementary
nitrogenous bases:
Adenine == Thymine
Cytosine == Guanine
The polynucleotide chains run in anti-parallel directions.
Structure of DNA molecule
The two polynucleotide chains
twist together to form a double
helix of DNA molecule.
Each turn of the double helix
consists of ten nucleotides.
Structure of DNA molecule
Summary
DNA molecule
TWO polynucleotide chains join by
H-bonds through complementary
nitrogeneous bases.
Each polynucleotide chain consists of
mononucleotides or nucleotides.
Each nucleotide is composed of three
parts: deoxyribose, phosphate group
and nitrogenous bases (A,T,C,G).
DNA duplication
The mode of DNA duplication is semi-conservative.
Actual happening during DNA duplication
Actual happening during DNA duplication
Actual happening during DNA duplication
Actual happening during DNA duplication
Actual happening during DNA duplication
Gene
Phenotype of a cell/organism depends on the type of
structural proteins synthesized and the type of functional
proteins(i.e. enzymes) produced.
The portion of DNA which stores the message
responsible for controlling the synthesis of a polypeptide
chain is called a gene.
Genetic Codes
AAA Phe
AAG Phe
AAT Leu
AAC Leu
AGA Ser
AGG Ser
AGT Ser
AGC Ser
ATT *
ATA Tyr ATG Tyr
ATC *
ACA Cys
ACG Cys
ACT *
ACC Try
GAA Leu
GAG Leu
GAT Leu
GAC Leu
GGA Pro
GGG Pro
GGT Pro
GGC Pro
GTA His GTG His
GTT Gln
GTC Gln
GCA Arg
GCG Arg
GCT Arg
GCC Arg
TAA Ile
TAG Ile
TAT Ile
TAC Met
TGA Thr
TGG Thr
TGT Thr
TGC Thr
TTA Asn
TTG Asn
TTT Lys TTC Lys
TCA Ser TCG Ser
TCT Arg
TCC Arg
CAA Val CAG Val
CAT Val CAC Val
CGA Ala
CGG Ala
CGT Ala
CGC Ala
CTA Asp
CTG Asp
CTT Glu
CTC Glu
CCA Gly
CCG Gly
CCT Gly
CCC Gly
Each code-word is represented by three nucleotides
called triplet code or codon.
Each triplet code stores the message recognizing a
specific amino acid.
The sequence of triplet codes along the
polydeoxyribonucleotide chain determines the specificity
of amino acids sequence along the polypeptide chain to
be synthesized.
What is the amino acid sequence of the polypeptide
chain synthesized by the portion of the DNA with
nucleotides TTTCGACCC?
Lys-Ala-Gly
Nonsense codons e.g. ATT, ATC
Some codons do not recognize any kind of amino acids.
They are called nonsense codons. They act as "starting
points" or "full stops" in the process of polypeptide
synthesis.
Degeneracy
Very often, two or more triplet codes may
recognize the same amino acid. Therefore the
codes contain more potential information than is
actually used by the cell. The code is therefore said
to be degenerated.
Non-overlapping code
Once the bases have been assigned to form a codon,
they will not be shared with other bases to form another
codon.
e.g.
GAAGCTGAC
If the code is
overlapping
Universal
It means the same code recognizes the same amino acid
is applicable in all kinds of organisms.
Gene mutation
Permanent, sudden change in nucleotides
sequence will change the sequence of amino
acids of the polypeptide chain to be synthesized,
therefore altering the genotype of a
cell/organism.
An example of gene mutation is sickle cell
anaemia.
Ribonucleic Acid (RNA)
The basic unit is ribonucleotide which is composed of
3 parts: ribose(5-C sugar), phosphoric acid, organic base.
P
CH2
O
Ribose
Organic base
The four organic bases are adenine, guanine, cytosine,
uracil(instead of thymine in DNA).
The ribose links by phosphodiester bond with phosphoric
acid forming the backbone structure while the organic
bases project as side chains.
There are three types of RNA:
messenger RNA
transfer RNA
ribosomal RNA
Protein synthesis
It consists of two main
stages:
1. Transcription:
copying the
genetic
information from
DNA in the form
of mRNA.
2. Translation:
conversion of the
message of the
mRNA into a
specific
polypeptide chain.
Transcription
A certain portion of the DNA molecule unwind.
The template strand of this part synthesizes a mRNA molecule using
the free ribonucleotides as raw materials.
The base sequence (codon sequence) of the mRNA is complementary
to that of the template strand of the DNA.
The mRNA then leaves the nucleus to the
ribosomes
Transfer RNA (tRNA)
Two important sites in this
molecule:
1. Amino acid binding site
2. Anticodon: This part has three
nucleotides which are used to
bind to the codon of the
mRNA.
Transfer RNA (tRNA)
There are 20 groups of tRNA.
Each is to transfer a specific
amino acid.
Relationship between the base
sequence of the anticodon and
the amino acid transferred.
Examples:
AAA Phe
AAG Phe
AAU Leu
AAC Leu
AGA Ser
AUA Tyr
ACA Cys
ACG Cys
The tRNA molecule binds with the amino acid
to form tRNA-amino acid complex.
These are the examples of tRNA-amino acid
complexes.
Translation – Ribosome moves along the mRNA to carry
out such process.
tRNA-amino acid complexes
use their anticodons to bind
with the complementary
codons along the mRNA. The
sequence of these complexes
Peptide bond is formed between the
is determined by the
sequence of the codons along two adjacent amino acid molecules.
the mRNA.
Translation
Direction of the
movement of the
ribosome
Translation
Polyribosomes or polysomes are used to increase the efficiency of
translation.
A number of ribosomes read the same mRNA molecule at the same
time. This enables the synthesis of a number of polypeptide
molecules within a short period of time.
Triplet codes of DNA
AAA Phe
AAG Phe
AGA Ser
AGG Ser
ATA Tyr ATG Tyr
ATT *
ATC *
ACA Cys
ACG Cys
TGA Thr
TGG Thr
TTA Asn
TTG Asn
TCA Ser TCG Ser
Anticodons of tRNA and the
amino acids they transfer
AAA Phe
AAG Phe
AGA Ser
AGG Ser
AUA Tyr
AUG Tyr
ACA Cys
ACG Cys
UGA Thr
UGG Thr
UUA Asn
UUG Asn
UCA Ser
UCG Ser
The template strand of a portion of the DNA molecule has the
following base sequence: TCGTTGATAAGGACAATT
1. What is the base sequence of the mRNA molecule transcribed?
2. What are the anticodons of the tRNA molecules bind to this
mRNA at the ribosome?
3. What is the sequence of the amino acids in the polypeptide to be
synthesized?