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Biology > Molecular Basis of Inheritance- 2 > Flashcards

Molecular Basis of Inheritance- 2 Flashcards

Replication, Transcription, Translation

1
Q

Where does DNA replication occur?

A

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

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2
Q

Name the enzymes involved in DNA replication

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

Function of Helicase in DNA replication

A

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

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4
Q

Function of Gyrase/Topoisomerase

A

prevents supercoiling of DNA

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5
Q

Function of SsBP

A

Helps in stabilizing the replication fork (SsDNA)

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6
Q

Function of Primase

A

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

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7
Q

Function of DNA polymerase

A

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

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8
Q

Function of DNA ligase

A

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

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9
Q

In which strand is synthesis continuous and discontinuous

A

Continuous in the leading strand
Discontinuous in the lagging strand

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10
Q

Why is polymerization discontinuous in the lagging strand

A
  • 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.
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11
Q

Differences between leading and lagging strand

A
  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

  1. Leading strand: No Okazaki fragments formed
    Lagging strand: Okazaki fragments formed
  2. Leading strand: 5’-3’ orientation of strand
    Lagging strand: 3’-5’ orientation of strand
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12
Q

What is transcription and where does it take place

A

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

It takes place in the nuclei

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13
Q

In which direction does transcription occur

A

5’-3’ direction

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14
Q

How is the transcription bubble exposed?

A
  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
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15
Q

In which direction are complementary nucleotides being attached on the mRNA

A

3’-5’ direction

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16
Q

What are exons and introns

A

Exons: Coding sequences
Introns: Non-coding sequences

17
Q

What are the non-coding and coding strands?

A

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

18
Q

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

A

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)

19
Q

Name the steps involved in transcription

A
  1. Initiation
  2. Elongation
  3. Termination
20
Q

Initiation step of transcription

A
  • RNA polymerase recognises and attaches to a small strip of DNA called the promoter on the template strand.
21
Q

Elongation step of transcription

A
  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
22
Q

Termination step of transcription

A
  • 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)

23
Q

What are spliceosomes? How do they perform their function?

A
  • 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
24
Q

How is the mRNA modified before getting translocated?

A
  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
25
Q

What is Lac operon

A

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

26
Q

What is an operon

A

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)

27
Q

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

A
  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
28
Q

Lac operon: When lactose is present in the medium

A

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

29
Q

Lac operon: When lactose is absent in the medium

A
  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)
30
Q

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

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

Biology (30 decks)

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