CHAPTER 6: LAC OPERON AND REVISION

Multiple Choice

Which of the following correctly represents the central dogma of molecular biology?

• DNA → Protein → RNA

• RNA → DNA → Protein

• DNA → RNA → Protein

• Protein → RNA → DNA

Multiple Choice

During the process of transcription, what molecule is synthesized?

• DNA

• mRNA

• Protein

• Amino acids

Multiple Choice

What is the role of mRNA in the central dogma?

• It carries genetic information from DNA to the ribosome for protein synthesis.

• It is the site of protein synthesis.

• It carries amino acids to the ribosome.

• It acts as an enzyme in protein synthesis.

Multiple Choice

Which of the following enzymes is responsible for unwinding the DNA double helix during replication?

• Primase

• DNA Polymerase

• Helicase

• Ligase

Multiple Choice

What is the main function of primase in DNA replication?

• Unwinds the DNA double helix

• Adds nucleotides to the growing DNA strand

• Synthesizes a short RNA primer

• Joins Okazaki fragments together

Multiple Choice

Which enzyme adds new nucleotides to a growing DNA strand during replication?

• Helicase

• Primase

• DNA Polymerase

• Ligase

Multiple Choice

Which of the following best describes the process of transcription?

• Conversion of mRNA into a polypeptide chain.

• Synthesis of a DNA strand from an RNA template.

• Synthesis of mRNA from a DNA template.

• Assembly of ribosomes in the cytoplasm.

Multiple Choice

In eukaryotic cells, where does translation occur?

• Nucleus

• Cytoplasm

• Mitochondria

• Endoplasmic reticulum

Multiple Choice

During transcription, the enzyme responsible for synthesizing mRNA is:

• DNA polymerase

• RNA polymerase

• Helicase

• Ligase

Multiple Choice

What is the role of a codon in the process of translation?

• It signals the start of transcription.

• It determines the sequence of amino acids in a protein.

• It initiates DNA replication.

• It marks the end of DNA synthesis.

Multiple Choice

Which of the following mRNA codons serves as a stop codon in protein synthesis?

• UAA

• AUG

• UUU

• GGG

LEARNING OUTCOMES

6.4 Gene regulation and expression – lac operon

a) Explain the concept of operon and gene regulation

(C3)

b) State the components of operon (C1)

c) Explain the components of lac operon and their

function in E.coli (C3)

d) Explain the mechanism of the operon in the

absence and presence of lactose (C3)

76

Operon

Learning Outcomes :

6.4 a) Explain the concept of operon and gene regulation

● A series of structural genes expressed as a group

controlled by the single operator and promoter

● Consists of promoter, operator and structural genes

● Structural genes function as a single transcription unit

transcribed into single mRNA

● Only in prokaryotes (e.g. bacteria)

Gene Regulation

Learning Outcomes :

6.4 a) Explain the concept of operon and gene regulation

● Genes

of

related

functions

are

grouped

into

one

transcription unit

● These genes are regulated such that they are all turned on

or off together

● These genes are coordinately controlled

● The on-off switch is a segment of DNA called an operator

Gene Regulation

Learning Outcomes :

6.4 a) Explain the concept of operon and gene regulation

● Operator can be switched off by repressor protein

● Repressor binds to operator and blocks the attachment

of

RNA

polymerase

to

the

promoter,

preventing

transcription of the genes

● The repressor is encoded by regulatory gene (outside

the operon, has its own promoter)

Component of Operon

Learning Outcomes :

6.4 b) State the components of operon

Operon includes the following:

1) Promoter
2) Operator
3) Structural genes; lacZ, lacY, lacA

lac operon

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

● The operon model was proposed by Francois Jacob

and Jacques Monod (1961) in E.coli

● Some of the bacterial genes will be expressed only

when it is necessary

● Lactose (lac) operon:


Regulates lactose metabolism in E.coli (contains
genes that encode for enzymes used in the
hydrolysis and metabolism of lactose)

Components of lac operon

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

● lac operon consists of:

1. Structural genes

2. Promoter

3. Operator

⮚lacZ
⮚lacY
⮚lacA

Components of lac operon

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

● lacZ : codes for /encodes β-galactosidase

● lacY : codes for /encodes permease

● lacA : codes for /encodes transacetylase

1. STRUCTURAL GENES

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

Components of lac operon

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

● Binding site for RNA polymerase

● Binding site for repressor protein

● Act as switch; which activate/ inactivate the operon

2. PROMOTER

3. OPERATOR

Learning Outcomes :

6.4 c) Explain the components of lac operon and their function in E.coli

● Encodes for repressor protein

● Located outside the operon, has its own promoter gene

REGULATORY GENE/ lacI

Learning Outcomes :

6.4 d) Explain the mechanism of the operon in the absence and presence of lactose

Mechanism of lac operon

LACTOSE ABSENT

Learning Outcomes :

6.4 d) Explain the mechanism of the operon in the absence and presence of lactose

Mechanism of lac operon

● Some / small amount of

lactose is converted to
allolactose (inducer)

● Allolactose binds to

repressor protein

● Repressor protein change

its conformation to become
inactive form

● Repressor protein cannot

binds to /detached from
operator

LACTOSE PRESENT

Learning Outcomes :

6.4 d) Explain the mechanism of the operon in the absence and presence of lactose

Mechanism of lac operon

● Promoter is unblocked

● RNA polymerase binds to

promoter

● lac operon switched on

● Transcription of structural

genes (lacZ, lacY and lacA)
occur

● β-galactosidase, permease

and transacetylase are
produced

● Lactose hydrolysed into

glucose and galactose

LACTOSE PRESENT

Learning Outcomes :

6.4 d) Explain the mechanism of the operon in the absence and presence of lactose

Mechanism of lac operon

LACTOSE PRESENT

Learning Outcomes :

6.4 d) Explain the mechanism of the operon in the absence and presence of lactose

One mRNA is translated into three polypeptides

Multiple Choice

What is the function of the lacZ gene in the lac operon?

• Encodes a protein that imports lactose into the cell

• Encodes the enzyme β-galactosidase, which breaks down lactose

• Encodes a protein that inhibits lactose breakdown

• Encodes the repressor protein that blocks transcription

Multiple Choice

Which molecule acts as the inducer in the lac operon?

• Glucose

• Allolactose

• Lactose permease

• Thiogalactoside transacetylase

Multiple Choice

What happens to the lac operon when there is no lactose present?

• The repressor binds to the operator, blocking transcription

• The structural genes are actively transcribed

• RNA polymerase binds to the promoter and initiates transcription

• The operon is induced, and enzymes for lactose metabolism are produced

Multiple Choice

Which of the following correctly describes the role of the lacI gene?

• Encodes β-galactosidase

• Encodes the repressor protein that regulates the lac operon

• Encodes a protein that facilitates the transport of lactose

• Encodes an enzyme that detoxifies by-products of lactose metabolism

Multiple Choice

Which gene produces the repressor protein that regulates the lac genes?

• lacZ

• lacY

• lacI

• lacA