Molecular Basis of Inheritance- 2

Question 1

Where does DNA replication occur?

Answer 1

DNA replication occurs in the nucleus in anti-parallel orientation at the “ori” site

Question 2

Name the enzymes involved in DNA replication

Answer 2

1. Helicase
2. DNA Gyrase/ Topoisomerase
3. SsBP- Single stranded Binding Proteins
4. Primase
5. DNA-dependent RNA-polymerase
6. DNA ligase

Question 3

Function of Helicase in DNA replication

Answer 3

Helps in separating or unwinding of the double stranded DNA (cutting the wweak hydrogen bonds between corresponding nucleotides).
This creates a replication fork

Question 4

Function of Gyrase/Topoisomerase

Answer 4

prevents supercoiling of DNA

Question 5

Function of SsBP

Answer 5

Helps in stabilizing the replication fork (SsDNA)

Question 6

Function of Primase

Answer 6

At ‘ori’ site
Acts as a recognition site for DNA dependent RNA polymerase with an exposed 3’-OH end

Question 7

Function of DNA polymerase

Answer 7

RNA dependent DNA polymerase- RNA dependent primer is the recognition site

• Carries out polymerisation and attaches complementary base pairs
• The direction of reading the template by the DNA polymerase is always 3’-5’
• The synthesis of new DNA on the template is always 5’-3’

DNA is antiparallel- reading is in one direction & attaching is in other

Question 8

Function of DNA ligase

Answer 8

Helps in joining or sealing (ligate) The nicks (gaps) formed during DNA replication

Question 9

In which strand is synthesis continuous and discontinuous

Answer 9

Continuous in the leading strand
Discontinuous in the lagging strand

Question 10

Why is polymerization discontinuous in the lagging strand

Answer 10

• Synthesis of the lagging strand has to be discontinuous because DNA polymerase cannot make a new strand in the 3’ to 5’ direction.
• Therefore, a series of Okazaki fragments (short DNA strands), each with their own RNA primer, must be made as to the fork advances.

Question 11

Differences between leading and lagging strand

Answer 11

1. Leading strand: One primer
   Lagging strand: Many primers
2. Leading strand: Continuous polymerisation
   Lagging strand: Discountinuous polymerisation

3.
Leading strand: Only one nick is sealed by the DNA ligase
Lagging strand: Several nicks are sealed by the DNA ligase

4. Leading strand: No Okazaki fragments formed
   Lagging strand: Okazaki fragments formed
5. Leading strand: 5’-3’ orientation of strand
   Lagging strand: 3’-5’ orientation of strand

Question 12

What is transcription and where does it take place

Answer 12

The process of formation of RNA from DNA is called transcription.

It takes place in the nuclei

Question 13

In which direction does transcription occur

Answer 13

5’-3’ direction

Question 14

How is the transcription bubble exposed?

Answer 14

1. Before the start of transcription, a short stretch of DNA (~14 bp) gets uncoiled with the help of the enzyme helicase
2. The supercoiling is prevented by topoisomerase/gyrase
3. The stability of uncoiled SsDNA is maintained by SsBP (Single stranded binding proteins)
4. This process exposes a transcription bubble

Question 15

In which direction are complementary nucleotides being attached on the mRNA

Answer 15

3’-5’ direction

Question 16

What are exons and introns

Answer 16

Exons: Coding sequences
Introns: Non-coding sequences

Question 17

What are the non-coding and coding strands?

Answer 17

Non-coding/anti-sense strand: The strand that has the polarity 3’-5’ and acts as a template

Coding/sense strand: The strand that has the polarity 5’-3’ is called the coding strand, even though it doesn’t code for anything i.e. it doesn’t get transcribed at all

Question 18

What is the logical reason for naming the coding strand this way

Answer 18

The coding strand sequence will be the same as that of the mRNA synthesised (except thymine which is replaced by uracil in the mRNA strand formed)

Question 19

Name the steps involved in transcription

Answer 19

1. Initiation
2. Elongation
3. Termination

Question 20

Initiation step of transcription

Answer 20

• RNA polymerase recognises and attaches to a small strip of DNA called the promoter on the template strand.

Question 21

Elongation step of transcription

Answer 21

1. The RNA polymerase moves along the template strand in 3’-5’ direction but adding nucleotides at the exposed 3’-OH end in the 5’-3’ direction
2. Synthesis of mRNA occurs in the 5’-3’ direction

Question 22

Termination step of transcription

Answer 22

• RNA polymerase stops transcription when it reaches the termination site
• It releases itself from the DNA strand and the new mRNA strand moves from the nucleus to the cytoplasm for translation (Eukaryotes)

The final mRNA strand contains both exons (coding sequence) and introns (non-coding sequence)

Question 23

What are spliceosomes? How do they perform their function?

Answer 23

• The structures that are responsible for removing the introns from the mRNA strand are called spliceosomes.
• They recognise the start and end sequence of introns and cut them and then re-ligate the exons to form a complete functional mRNA consisting of only coding sequences (exons)
• THis sequence is then translocated to the cytoplasm where translation takes place

Question 24

How is the mRNA modified before getting translocated?

Answer 24

1. The 5’ end of mRNA is capped with a modified guanosine triphosphate, and it is called 5’ capping
2. Poly A tail (a sequence of adenine residues) is also attached to the 3’ end of mRNA to avoid nuclease attack

Question 25

What is Lac operon

Answer 25

Lac operon is an operon or a group of genes with a single promoter that encode genes for the transport and metabolism of lactose in E.coli and other bacteria

Question 26

What is an operon

Answer 26

An operon is a DNA segment containing a few genes that serve to regulate a particular metabolic acitivity. THey are usually expressed as a particular unit (transcribed together)

Question 27

Name the structural genes IN ORDER + what they code for + function

Answer 27

1. lacZ
   codes for: β-Galactosidase
   function: hydrolyze lactose into glucose and galactose
2. lacY
   codes for: Permease
   function: Lactose transporter (into cell)
3. lacA
   codes for: Transacetylase
   function: Adds acetyl group to galactose

Question 28

Lac operon: When lactose is present in the medium

Answer 28

RNA polymerase binds to the Promoter site and the three structural genes are transcribed, Operon is ON

Question 29

Lac operon: When lactose is absent in the medium

Answer 29

1. A repressor protein binds to the Operator site seen between the Promoter site and Structural gene Z (lacZ)
2. This prevents the RNA polymerase from moving along the structural genes and they are not transcribed (Operon is OFF)

Question 30

Lac operon: When both glucose and lactose are present in the medium

Answer 30

1. Glucose is metabolized first
2. Operon is switched on subsequently for the metabolism of lactose