LECTURE NOTE WEEK 9 & 10

SB015 BIOLOGY LECTURE

Chapter 6 :

Expression of
Biological
Information

6.1
DNA and
Genetic
Information

Learning Outcomes

By the end of this lesson, students should be able to:
a) State the concept of Central Dogma

Central Dogma

1) In general, genetic information flows from DNA to RNA
to protein.

1) This concept was called the “Central Dogma” by
Francis Crick in 1956.

Central Dogma

3) Genes provide the instructions for making
specific protein, but a gene does not build a
protein directly.

3) The expression of genes includes two stages:
a) Transcription
b) Translation

Central Dogma

5) Genes will synthesis mRNA as
genetic message via transcription.

5) Then, mRNA will be translated to
produce specific enzymes and
other protein, via translation.

DNA Replication by Semiconservative model

1) DNA replication is a process
occur in nucleus where DNA
could be precisely copied.

1) Each strand of DNA molecule
could serve as a template for
the synthesis of the new
daughter strands.

DNA Replication by Semiconservative model

3) Each of the template strand will pair with their complementary nucleotides to
replace its missing partner.

3) Each newly synthesised DNA molecules produced are consists of one old
strand and one new strand.

DNA Replication : Overview of DNA structure

Enzymes and
Proteins involved
in DNA Replication

6.3

Protein Synthesis
: Transcription
and Translation

Learning Outcomes

By the end of this lesson, students should
be able to:

b) Introduce codon and its
relationship with sequence of amino
acid using genetic code table.

Codon

❏ Triplet nucleotides in
mRNA.

❏ Read in 5’ to 3’ direction.
❏ Also used for DNA

nucleotide triplets along
nontemplate strand.

Genetic Code

● Genetic code is a code which
specifies the relationship
between the sequence of codon
in the mRNA and the sequence of
amino acid in a polypeptide

Features of Genetic Code

● The code is read in groups of three
nucleotides / triplet bases on mRNA. It is
known as codon.

● Genetic code is nearly universal in all living
organism.

● Genetic code is said to be degenerate /
redundant– means that more than one
codon can specify the same amino acid

Features of Genetic Code

● The are 64 possible
combination of triplet bases.

● Non-overlapping- every codon
will code one specific amino
acid

● Has Start codon : AUG

● Has 3 Stop codons: UAA, UAG,
UGA (do not code for any
amino acid)

Start and Stop Codon

6.4

Gene Regulation
and Expression

(lac operon)

Learning Outcome

By the end of this lesson, students should be able to:
b) State the components of operon

Concept of Operon

1) In prokaryotic cell, protein synthesis is controlled by operon
system.

1) An operon is a coordinately regulated transcription unit in
bacteria which consists of a promoter and structural genes.

What is an operon?

❖ A single genes controlled by single promoter
❖ A unit made up of cluster of genes

An operon consists of promoter, operator and a cluster of genes (Campbell 12thEdition)
❖ OR

❖ •Group of structural and regulating genes that function as a single unit (Mader 12th Edition)

(Campbell 12th Edition)

Concept of operon

★ Example for operon: lac operon (lac stands for ‘lactose’)
★ Gene regulation and expression occur at the transcription stage. During translation it will

produce the polypeptides which act as the enzymes to hydrolyse lactose from its host.

Concept of operon

1) Consist of structural genes (lacZ, lacY, and lacA), operator
and promoter. The operon is regulated by the product of
regulatory gene (lacI)

transcription
translation

Concept of operon