DNA molecule

Gene 1

Gene 2

Gene 3

DNA strand

TRANSCRIPTION

RNA

Polypeptide

TRANSLATIONCodon

Amino acid

Central Dogma of Biology How does the information flow in biological

systems?

• DNA copy itself:• Replication

• DNA synthesize RNA• Transcription

• RNA synthesize protein• Translation

Two Main Process: Transcription and Translation

DNA (deoxyribonucleic

acid)

RNA (Ribonucleic acid)

Two strands forming a double helix

One strand only

Deoxyribose Ribose

A, C, G and T A, C, G and U

H

3.5.1 Compare the structure of RNA and DNA

3.5.2 DNA Transcription1. 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

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.

3.5.2 DNA Transcription

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 <20• 2 letter code 4 x 4 = 16 AAs <20• 3 letter code 4 x 4 x 4 = 64 AAs >>20

3. 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).

1. Triplete Code2. Degenerated3. Universal

3.5.3The Genetic Code

Nucleic Acids: RNA types

There are 3 different types of RNA:

1. Messenger RNA: Transcription1. Single strand2. Transport nucleus – Cytoplasm

2. Ribosomal RNA1. Structural & Functional component of Ribosomes

3. Transfer RNA: Translation1. 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.

T A C C G A A T G G C C

A C CG AG GCUU U G

Metionine Alanine Tyrosine Arginine

DNA

mRNA

Polypeptide:

Translation take place on cell structures called Ribosomes, using mRNA & tRNA.

Ribosomes are in the cytoplasm, outside the cell nucleus.

3.5.4 DNA Translation:

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

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: 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.