Spectrum Biology - September 2016

7 Genes are Transcribed into RNAs

The portion of gene that is replicated as RNA is called the coding

sequence. In eukaryotes, the coding sequence is often broken up

into segments, called exons, separated by noncoding sequences

called introns. The process of synthesising an RNA molecule is

called transcription. It occurs in the following steps:

Transcription

1. Part of the DNA double helix of one chromosome.


2. RNA polymerase ‘recognises’ a ‘start’ signal and attaches to DNA
   at this point. Complementary
   noncoding strand

‘Start signal

on coding

strand of DNA

Direction of

transcription

RNA polymerase

Hydrogen bonds linking the two strands are broken; the double

helix unwinds in the region for transcription (a gene)

3. RNA polymerase molecule travels along the DNA strand, catalysing
   the production of messenger RNA molecules.

Messenger

RNA

‘Stop’ signal

RNA polymerase

condenses

nucleotides

together

Messenger

RNA strand

Pool of

nucleotides

Coding strand

of DNA

4. After transcription is completed, the DNA returns to the double-stranded form. Base pairing


5. DNA molecules in eukaryotes
   are so long that their length
   cannot be replicated from
   only one initiation point;
   instead, replication points
   open up at various sites
   along the chromosome.

Parent DNA

strands, about

to be replicated

Building

units

New chain

Template

2. Enzyme (DNA polymerase)
   unzips parent strands and
   adds nucleotides.


3. The nucleotides for building the new
   DNA strands are delivered as :
   Adenosine Triphosphate (ATP)
   Guanosine Triphosphate (GTP)
   Thymidine Triphosphate (TTP)
   Cytidine Triphosphate (CTP)
   At the replication site the two terminal phosphates are removed
   as the nucleotide is added to the DNA chain.

P

P

P

P

PP

Deoxyribose

+ Phosphate

Complem-

entary

strands

Parent DNA

molecule

Replication

begins

Replication sites

join up

DNA Replication

8 Messenger RNAs are Translated to form Proteins

1. Translation begins when the large and
   small ribosome subunits, each containing
   rRNA, combine with tRNA and mRNA at
   the first (start) codon on the mRNA.

Ribosome

2. Elongation occurs as the
   ribosome moves from the
   first codon to the next,
   attracting a second tRNA with
   the correct anticodon
   sequence and carrying a new
   amino acid. The two amino
   acids are joined together,
   Amino elongating the polypeptide.
   acid

Large

subunit

Start codon

AUG

Small subunit

Ribosomal RNA

mRNA

Stop codon

UAG

5 ′ 3 ′

5 ′

3 ′

5 ′ 3 ′

3. Termination occurs when a protein
   called release factor attaches to
   the last (stop) codon, releasing
   the polypeptide.


4. The released polypeptide either folds into
   a protein or becomes a subunit of a multi-
   subunit protein, while the ribosome and
   mRNA are recycled for another round
   of translation.
   Different steps of translation in eukaryotes

Release factor

5 ′ 3 ′

tRNA

 The combined product

of rRNAs, tRNAs and

mRNAs working

together is a newly

synthesised

polypeptide.

 During this process,

three nucleotide

codons in the coding

sequence of mRNA are

matched with

anticodons in

tRNA by ribosomes to

determine the

sequence of amino

acids in the

resulting polypeptide.

 The process of

converting the

ribonucleotide

sequence of mRNA into

an amino acid sequence

in a polypeptide is

called translation.

 Its various steps are

shown in the figure

given.