Genes in Action

Aims:

Must be able to recall the main structures of DNA and
how the molecule is put together.

Should be able to define the terms transcription and
translation and what the purpose of the process is.

Could be able to outline the processes of transcription
and translation.
Transcription - Background

DNA is located in the Nucleus.

The information contained in the DNA is
copied into RNA and exported into the
cytoplasm where the process coded for
can be completed.

The form of RNA that carries the
information is messenger RNA (mRNA).
Transcription - Basics

DNA – mRNA

Requires:
– Energy + RNA nucleotides (ATP, CTP, GTP,
UTP)
– DNA helicase (unwind DNA)
– RNA polymerase (reads DNA)
Transcription - Process

DNA unwound by DNA helicase.

3’-5’ strand (transcription strand) read by RNA polymerase.

RNA polymerase synthesises a complimentary 5’-3’ RNA strand
(using law of base pairing).

Diagram:

Primary product of transcription is pre-mRNA.

Introns are cut from pre-mRNA and a poly-A tail added (posttranscription modification).

Diagram:

At completion of transcription RNA strand leaves nucleus through
nuclear pore.
Alternative Splicing of Pre-mRNA

‘One gene – one polypeptide’ – How can a relatively small number
of genes produce such complexity.

New research shows that one gene can be regulated in different
ways so that more than one protein can be produced:
– One gene can produce a different protein at different stages of
development.
– One gene can produce one protein in one tissue and a different protein
in another tissue.

Estimated that approx 30% of Human genes regulated in this way.

Triggered by ALTERNATIVE SPLICING of Pre-mRNA molecules in a
single gene.

Diagram (p395)
Translation - Background

mRNA moves from nucleus to cytoplasm.

Process happens at ribosomes.

Ribosomes read the mRNA and convert the
sequence of codons to a sequence of amino
acids.

Amino acids released to form proteins.
Translation - Basics

mRNA – Amino Acid sequence

Requires:
– tRNA
– Ribosomes
– Amino Acids
– Synthetase enzymes
– ATP
Translation - Process

tRNA structure:

Diagram:

Highly specific synthetase enzymes ensure that correct Amino Acids are
bound to tRNA molecule with matching anti-codon.

Binding of Amino Acid to tRNA molecule requires ATP.

Ribosome reads triplet code on mRNA and catylases protein synthesis.

Diagram:

Each sub-unit is composed of a complex of proteins and ribosomal RNA.

Ribosome reads the mRNA and has two binding sites for tRNA.

Diagram:

Once the A-site is occupied by tRNA molecule and it’s Amino Acid, peptide
chain is transferred from P-site to A-site.

Diagram:

tRNA molecule in P-site leaves ribosome.

tRNA molecule reactivated by binding to appropriate Amino Acid.

P-site is now empty.

Ribosome moves along 3 bases. tRNA with growing peptide chain now
occupies the P-site.

Diagram:

A-site is now empty and can receive next activated tRNA with it’s Amino Acid.

Process of building up Amino Acid chain = ELONGATION.

Ribosome continues until stop codon occupies A-site.

Peptide chain released from ribosome to take up tertiary structure.
Polysome (Polyribosome)

Length of mRNA with many (100+)
ribosomes attached to it.

Importance: Allows hundreds of copies
of a particular peptide to be made in short
period of time.
Prokaryotes v Eukaryotes
Prokaryotic DNA
Eukaryotic DNA
Main chromosome is a circular
molecule of DNA - PLASMID
Chromosome is linear molecule of
DNA
DNA naked
DNA is complexed with histone
proteins
DNA comprises unique nucleotide
sequences
DNA contains many repeated
sequences
DNA free within cell
DNA enclosed within nucleus
Coding sequences of genes are
uninterrupted
Coding sequences (exons) are
interrupted by introns
Additional DNA in form of plasmids
present
Plasmids are absent
Both undergo transcription and translation.
 Eukaryotes transcribe in nucleus and translate in cytoplasm, prokaryotes
both in cytoplasm – faster

Activity

Answer the questions on the sheet.

Answer the questions from pages 198 to
202 in the Biozone book.