Molecular Basis of Inheritance- 1

Question 1

DNA structure

Answer 1

1. 4 nitrogen bases
2. 1 pentose sugar
3. phosphate group

4 nitrogen bases + 1 pentose sugar: nucleoside
nucleoside + phosphate group: nucleotide

Question 2

What did Griffith use in his experiment?

Answer 2

Frederick Griffith observed two strains of Diplococcus pneumoniae:
1. Virulent smooth (S) type
2. Non-virulent rough (R) type

Question 3

What did Griffith inject the mice with and what did he observe?

Answer 3

1. Live S type
   Observation: Suffered pneumonia and died
2. Live R type
   Observation: Suffered no illness and survived
3. Heat killed S-type
   Observation: Suffered no illness and survived
4. Mixture of R-type and heat killed S-type
   Observation: Some mice suffered pneumonia and died

Question 4

What was Griffith’s conclusion?

Answer 4

Griffith concluded that the heat-killed bacteria somehow converted live avirulent cells to virulent cells, and he called the component of the dead S-type bacteria the “transforming principle”.

Question 5

What was the Avery-MacLeod-McCarty experiment?

Answer 5

1. They found that DNA isolated from heat-killed S-cells when added to R-cells changed their surface character from rough to smooth and also made them pathogenic.
2. When the extract was treated with DNAase, the transforming ability was lost.
3. When the extract was treated with Protease, the transforming ability wasn’t affected

This suggested that DNA was the transforming factor and not proteins

Question 6

What was the Hershey-Chase experiment

Answer 6

1. Protein coat of bacteriophage radiolabelled with S³⁵
2. DNA labelled with P³²

Attachment of phages to host cells
After removal of the phage coats it was observed:

1. Cell containing little S³⁵ labelled protein + unlabelled DNA
2. Cell containing P³² labelled DNA

Provided convincing evidence that DNA is the genetic material

Question 7

Why was the bacteriophage chosen for the Hershey-Chase experiment?

Answer 7

1. When it infects the bacteria, it releases only the genetic material into the bacterial cell and the protein coat is left behind in the culture medium
2. P group is seen only in the DNA and S group only in the protein coat

Question 8

Why was E. coli chosen for the Hershey-Chase experiment?

Answer 8

It has the shortest generation time of 20 minutes

Question 9

Why is DNA said to be anti-parallel in orientation?

Answer 9

DNA strands run in opposite directions

Question 10

How are the strands held together

Answer 10

By the weak hydrogen bonds that hold the nitrogen bases together

Question 11

Chargaff Rule

Answer 11

A (purine) pairs with T (pyrimidine)
G (purine) pairs with C (pyrimidine)

A% = T%
G% = C%

Amount of purine= Amount of pyrimidine

Ratio of A+T/G+C is the same in a particular species but can vary in different species

Question 12

Length of one complete turn of a double helix

Answer 12

3.4 nm

Question 13

No. of nucleotide pairs in one complete turn

Answer 13

10

Question 14

Distance bw two successive bp

Answer 14

0.34 nm

Question 15

Diameter of the helix

Answer 15

2nm

Question 16

Width of major and minor groove

Answer 16

Major groove: 22 Angstrom
Minor groove: 12 Angstrom

Question 17

Who confirmed the semi-conservative nature of the DNA

Answer 17

Meselson and Stahl

Question 18

Meselson Stahl experiment method

Answer 18

• N-15 (heavy) and N-14 (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in CsCl
• Bacteria are grown in N-15 (heavy) medium
• Some bacteria transferred to N-14 (light) medium and bacterial growth continues
• Take samples after 0 minutes, 20 minutes (after 1 round of replication), and 40 minutes (after 2 rounds of replication)

Question 19

Meselson Stahl experiment results

Answer 19

After 2 generations, half the DNA was intermediate and half was light-only; there was no heavy-only DNA

Question 20

Meselson-Stahl experiment conclusion

Answer 20

This pattern could only have been observed if each DNA molecule contains a template strand from the parental DNA. Thus, DNA replication is semiconservative.

Question 21

Different types of RNA

Answer 21

1. tRNA- transfer RNA
2. rRNA- ribosomal RNA
3. mRNA- messenger RNA

Question 22

Size and fn of tRNA

Answer 22

Size: small
Function: transports amino acids to site of protein synthesis

Question 23

Size and fn of rRNA

Answer 23

Size: Several kinds- variable in size
Function: Combines with proteins to form ribosomes, the site of protein synthesis

Question 24

Size and fn of mRNA

Answer 24

size: variable
Function: Directs amino acid sequence of proteins

Question 25

Codon

Answer 25

A codon is a DNA or RNA sequence of three nucleotides that forms a unit of genomic information encoding a particular amino acid or signaling the termination of protein synthesis (stop signals)

Question 26

Initiation/Start Codon

Answer 26

AUG
Codes for methionine

Question 27

Stop/termination codons

Answer 27

UAA, UAG, UGA

Question 28

Who decoded the genetic code

Answer 28

H. G. Khorana

Question 29

GRW Thymine (nitrogen base) is absent in a genetic code

Answer 29

A genetic code is made up of mRNA- where uracil is present in place of thymine

Question 30

Salient features of genetic code

Answer 30

1. Universal- codons code for any amino acid in any organism, be it a bacterium or human being
2. Non-ambiguous- each codon codes for only one amino acid, so the genetic code is unambiguous and specific
3. Comma-less- Codons are read in 5’3’ direction and no punctuation
4. Degenerate- Some amino acids are coded by more than one codon
5. Non-overlapping- 3 successive nitrogen bases code for only one amino acid

Question 31

Shine-Dalgarno sequence

Answer 31

Ribosomal binding site in bacterial and archaeal messenger RNA, generally located around 8 bases upstream of the start codon AUG