Chapter 6

Nature and Action of the Gene

O

CH2

OHOH

HH

P

Organic base

Structure of DNA molecule

The basic unit or monomer to build up a DNA molecule is called deoxyribonucleotide or nucleotide. It consists of three parts:

1) Deoxyribose

2) Phosphate group

3) Organic base or nitrogenous base

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

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 duplicationDNA 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

GeneGene

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

ATA Tyr ATG Tyr ATT * 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 codetriplet 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.

O

CH2

P

Organic base

Ribose

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 synthesisProtein 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 is determined by the sequence of the codons along the mRNA.

Peptide bond is formed between the two adjacent amino acid molecules.

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.

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

Triplet codes of DNA

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

Anticodons of tRNA and the amino acids they transfer

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?