Central Dogma of Biology
 How does the information flow in biological systems?
DNA
molecule
Gene
1
Gene 3
Gene 2
DNA strand
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Amino acid
Two Main Process:
Transcription and Translation
• DNA copy itself:
• Replication
• DNA synthesize RNA
• Transcription
• RNA synthesize protein
• Translation
3.5.1 Compare the structure of RNA and
DNA
DNA
(deoxyribonucleic acid)
RNA
(Ribonucleic acid)
Two strands forming a
double helix
One strand only
Deoxyribose
Ribose
H
A, C, G and T
A, C, G and U
3.5.2 DNA Transcription
1.
DNA helix unzips
2.
One of the 2 DNA strands acts as a template
3.
RNA nucleotides line up along one strand of the DNA following the base-pairing rules.
4.
No T, Instead U
3.5.2 DNA Transcription
5. RNA polymerase forms covalent bonds between nucleotides.
6. Transcription continues until one entire gene has converted to RNA.
7. The single-stranded messenger RNA separate and the DNA strands rejoin.
3.5.3The Genetic Code
How do nucleotides specify 20 amino acids?
1.
4 different nucleotides (A, G, C, T)
2.
Possible codes:
•
1 letter code  4 AAs
•
2 letter code  4 x 4 = 16 AAs
•
3 letter code  4 x 4 x 4 = 64 AAs
3.
<20
<20
>>20
Three letter code with 64 possibilities for 20 amino acids suggests that
the genetic code is degenerate (i.e., more than one codon specifies the
same amino acid).
3.5.3The Genetic Code
1. Triplete Code
2. Degenerated
3. Universal
Nucleic Acids: RNA types
There are 3 different types of RNA:
1. Messenger RNA: Transcription
1. Single strand
2. Transport nucleus – Cytoplasm
2. Ribosomal RNA
1. Structural & Functional
component of Ribosomes
3. Transfer RNA: Translation
1. Bring aa to Ribosomes for protein
assembly
3.5.4 DNA Translation:
1. The base sequence of mRNA is used as a guide for assembling the sequence of amino
acids that will be a polypeptide.
2. This process of protein production using a mRNA as a guide is called TRANSLATION.
DNA
T A C
C G A
A T G
G C C
mRNA
A U G
G C U
U A C
C G G
Polypeptide: Metionine
Alanine
Tyrosine
Arginine
3.5.4 DNA Translation:
Translation take place on cell structures called Ribosomes, using mRNA & tRNA.
Ribosomes are in the cytoplasm, outside the cell nucleus.
Transfer RNA:
1.
Amino acids are carried to the ribosomes by
smaller RNA molecules called transfer RNA
(tRNA).
2.
Single strands folded back on themselves:
“Clover Leaf structures”
3.
Role = Carry aa to the ribosomes
-aminoacid attachment site
-anticodon –codon attachment site
3.5.4 DNA Translation: Process
1. mRNA binds the small subunit of the ribosome
2. tRNA molecules are present, each one carrying the specific aa & anticodon
3. tRNA binds to the ribosome at the site where the anticodon matches the codon on the
mRNA
4. Two tRNAs binds at once and the first one in transfers the growing polypeptide chain to
the second one in.
5. The ribosome moves along the mRNA and the process continues until a stop codon is
reached when the polypeptide is realeased.
3.5.4 DNA Translation: Process
3.5.4 DNA Translation: Overview
3.5.5 One Gene – One Polypeptide Theory
One gene is transcribed and translated to produce one polypeptide.
Some protein are composed of a number of polypeptides and in this theory each
polypeptide has its own gene.
e.g. haemoglobin is composed of 4 polypeptides (2 of each type)
and there is a gene for each type of polypeptide.
This theory, like so many in biology has exceptions.
1) Some genes code for types of RNA which do not produce polypeptides.
2) Some genes control the expression of other genes.