Genetic code and transcription

Genetic code

information carried in the DNA specifies the protein end product

Transcription

conversion of genetic information from DNA to RNA

Translation

the information from an
mRNA is translated to the amino acid
sequence of a protein

The genetic code is written in

linear form, using the ribonucleotide bases of mRNA as “letters”

Codon

Each “word” consists of three ribonucleotide bases (triplet)

The code contains:

one start and three stop triplets that initiate
and terminate translation

The genetic code is:

unambiguous

degenerate

commales

non-overlapping

nearly universal

Unambiguous

each triplet specifies only a single amino acid

Degenerate

a given amino acid can be specified by more
than one triplet codon. This is the case for 18 of the 20 amino acids

initiate

terminate

Commaless

-No comma 

-No breaks

Nonoverlapping

any single ribonucleotide at a specific location within the mRNA is part of only one triplet

Universal

Only minor exceptions, almost all viruses,
bacteria, archaea, and eukaryotes use a single coding dictionary.

Proteins are composed of

Amino acids linked by peptide bonds

One start codon

AUG

Three stop codons

UAA
UAG

UGA

Initiation codon:

-The initial codon is usually AUG.
-It specifies the amino acid methionine

Open reading frame (ORF):

A reading frame set by the initiation
codon and ended with a stop codon

( There can be several open reading frames in one mRNA strand)

Codons are

commaless and nonoverlapping

Different points of transcription initiation lead to:

Overlapping genes

FRAMESHIFT MUTATIONS

mutation:adding or subtracrting nucleotides

THE GENETIC CODE IS NEARLY

Universal

RNA synthesis uses

ribonucleotides instead of
deoxyribonucleotides

No primer is required

For transcription initiation

RNA polymerase directs the synthesis of RNA using

a DNA template

TRANSCRIPTION PROCESS

• Initiation
• Elongation
• Termination

Transcription begins with RNA polymerase binding DNA at the

promoter sequence.

Promoter

segment of DNA upstream (5’ direction) of the gene to be
transcribed that recruits RNA polymerase

The σ (sigma) factor

Responsible for promoter recognition.

Transcription start site

point at which transcription begins

transcription occurs in the

5’ to 3’ direction

Template strand

The strand of DNA that is used to form a complementary RNA molecule

Two cis-acting elements (consensus
sequences) are recognized by the sigma
factor:

1. -10 region 

2. -35 region