Molecular Basis of Inheritance- 1 Flashcards
DNA structure
- 4 nitrogen bases
- 1 pentose sugar
- phosphate group
4 nitrogen bases + 1 pentose sugar: nucleoside
nucleoside + phosphate group: nucleotide
What did Griffith use in his experiment?
Frederick Griffith observed two strains of Diplococcus pneumoniae:
1. Virulent smooth (S) type
2. Non-virulent rough (R) type
What did Griffith inject the mice with and what did he observe?
- Live S type
Observation: Suffered pneumonia and died - Live R type
Observation: Suffered no illness and survived - Heat killed S-type
Observation: Suffered no illness and survived - Mixture of R-type and heat killed S-type
Observation: Some mice suffered pneumonia and died
What was Griffith’s conclusion?
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”.
What was the Avery-MacLeod-McCarty experiment?
- 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.
- When the extract was treated with DNAase, the transforming ability was lost.
- When the extract was treated with Protease, the transforming ability wasn’t affected
This suggested that DNA was the transforming factor and not proteins
What was the Hershey-Chase experiment
- Protein coat of bacteriophage radiolabelled with S³⁵
- DNA labelled with P³²
Attachment of phages to host cells
After removal of the phage coats it was observed:
- Cell containing little S³⁵ labelled protein + unlabelled DNA
- Cell containing P³² labelled DNA
Provided convincing evidence that DNA is the genetic material
Why was the bacteriophage chosen for the Hershey-Chase experiment?
- 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
- P group is seen only in the DNA and S group only in the protein coat
Why was E. coli chosen for the Hershey-Chase experiment?
It has the shortest generation time of 20 minutes
Why is DNA said to be anti-parallel in orientation?
DNA strands run in opposite directions
How are the strands held together
By the weak hydrogen bonds that hold the nitrogen bases together
Chargaff Rule
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
Length of one complete turn of a double helix
3.4 nm
No. of nucleotide pairs in one complete turn
10
Distance bw two successive bp
0.34 nm
Diameter of the helix
2nm
Width of major and minor groove
Major groove: 22 Angstrom
Minor groove: 12 Angstrom
Who confirmed the semi-conservative nature of the DNA
Meselson and Stahl
Meselson Stahl experiment method
- 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)
Meselson Stahl experiment results
After 2 generations, half the DNA was intermediate and half was light-only; there was no heavy-only DNA
Meselson-Stahl experiment conclusion
This pattern could only have been observed if each DNA molecule contains a template strand from the parental DNA. Thus, DNA replication is semiconservative.
Different types of RNA
- tRNA- transfer RNA
- rRNA- ribosomal RNA
- mRNA- messenger RNA
Size and fn of tRNA
Size: small
Function: transports amino acids to site of protein synthesis
Size and fn of rRNA
Size: Several kinds- variable in size
Function: Combines with proteins to form ribosomes, the site of protein synthesis
Size and fn of mRNA
size: variable
Function: Directs amino acid sequence of proteins
Codon
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)
Initiation/Start Codon
AUG
Codes for methionine
Stop/termination codons
UAA, UAG, UGA
Who decoded the genetic code
H. G. Khorana
GRW Thymine (nitrogen base) is absent in a genetic code
A genetic code is made up of mRNA- where uracil is present in place of thymine
Salient features of genetic code
- Universal- codons code for any amino acid in any organism, be it a bacterium or human being
- Non-ambiguous- each codon codes for only one amino acid, so the genetic code is unambiguous and specific
- Comma-less- Codons are read in 5’3’ direction and no punctuation
- Degenerate- Some amino acids are coded by more than one codon
- Non-overlapping- 3 successive nitrogen bases code for only one amino acid
Shine-Dalgarno sequence
Ribosomal binding site in bacterial and archaeal messenger RNA, generally located around 8 bases upstream of the start codon AUG
