Flow of Genetic Information

Question 1

Central Dogma of Molecular Biology

Answer 1

The information content of DNA is in the form of nucleotides
* DNA inherited by an organism produces specific traits by dictating the synthesis of proteins
* Proteins are the link between genotype and phenotype via gene expression
* Gene expression = the process by which DNA directs protein synthesis

Question 2

One gene - one enzyme hypothesis (1941)

Answer 2

• Cells synthesize and degrade molecules in a series of steps, termed a metabolic pathway
• Different enzymes catalyze different steps along the pathway. When the genes coding for those enzymes are ‘knocked out’ by mutation, the pathway is interrupted at that step

Question 3

One gene-one enzyme hypothesis:

Answer 3

each gene dictates the production of a specific enzyme
* But not all proteins are enzymes so revised to one gene-one protein hypothesis

Question 4

One gene-one polypeptide hypothesis (1945)

Answer 4

Many proteins are composed of several polypeptides, so now restated as one gene-one polypeptide hypothesis

Question 5

The concept: From DNA to Protein via gene
expression

Answer 5

• Comprises two stages: transcription and translation
• Transcription is the synthesis of RNA under the direction of DNA; produces messenger RNA (mRNA)
• Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA; takes place on ribosomes
  G1 + G2 – growth phases, metabolic functioning of cells mostly occurs in G1

Question 6

In a bacterium (prokaryotes)…

Answer 6

mRNA produced via transcription and immediately
translated without any further processing

Question 7

In eukaryote cells…

Answer 7

• transcription and translation are compartmentalised
  *RNA transcripts are first produced and then modified within the nucleus – RNA processing
• transported into the cytoplasm and translated on ribosomes

Question 8

Twenty amino acids

Answer 8

make up the building blocks of protein (all deciphered by 1960s), but there are only four nucleotide bases in DNA

Question 9

Triplet code:

Answer 9

a series of non-overlapping, three-nucleotide words; termed codons

Question 10

The genetic code

Answer 10

• Of the 64 triplets, 61 code for amino acids; 3 triplets are ‘stop’ signals to end translation
• Genetic code is redundant but not ambiguous
• NO codon specifies more than one amino acid
• BUT each amino acid can be encoded by more than one codon (except AUG – Met)

Question 11

Translation of the code

Answer 11

• Template strand - one of the two DNA strands provides
  the template for transcription
• During translation, the mRNA codons are read in the 5’ to 3’ direction
• Translation is initiated at a start codon and terminated at a stop codon
• These codons set the reading frame

Question 12

Reading frame

Answer 12

Codons must be read in the correct reading frame (correct groupings) for the specified polypeptide to be correctly produced
STT – START Methionine (AUG)
STP – STOP (UAA, UAG, UGA)

Question 13

Evolution of the genetic code

Answer 13

• The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
• Genes can be transcribed and translated after being transplanted from one species to another – genetically modified
• Luciferase…… luciferin +O2→oxyluciferin + light