Genes in

Action

Gene Function

• Structural genes– Become part of the structure & functioning of an organism

• Regulatory genes– Control the action of other genes– ie. “switch genes on or off”– Or control rate of production of proteins– Can switch genes on or off in one of two ways

Two types of gene regulation

• Regulatory genes may code for a DNA-binding protein– These have a positively charged binding site that will

enable it to bind to DNA– They will bind to a region near a gene and directly turn it

on or off

• Regulatory genes may code for a signalling protein– This will bind to a receptor on the cell membrane– Genes will be turned on or off via signal transduction

Homeotic Genes

• “Master genes” that control embryonic development in insects and vertebrates.

• A malfunctioning homeotic gene in flies may result in wings, legs, antennae and halteres being absent, or appearing in places that they should not.

Homeotic Genes

• In humans, homeotic genes fall in to 4 groups (Hox A,B,C & D).

• These encompas 39 genes spread over 4 chromosomes

• Malfunction of HOXC8 results in an extra pair of ribs• Malfunction of HOXD13 results in an extra digit

between digits 3 & 4 (often fused)

Gene Structure• The arrangement of base pairs (c) in a piece of

double stranded DNA (d) will determine many things, such as the coding and non-coding portions of DNA (a), and the length of a gene (b). Thus many representations of the same strand required.

Gene Structure

• Enzymes need to know when to start and stop reading a section of DNA.

• If the base pairs were a sentence, regulatory genes may be likened to capital letters and full stops.5’

5’3’

3’

Regulatory gene

START

STOP

Promoterregion

Terminator region

Gene Expression

• Our DNA is like the master plan for building an organism

• Genes are specific instructions on how to build one tiny part of the entire organism.

• Genes are located on the DNA, in the nucleus of our cells

• The mechanisms for making the products for which these genes code are in the cytoplasm

• How does the message get out of the nucleus and in to the cytoplasm?

Gene Expression• In order to be expressed, DNA must be

transcribed in to mRNA.

A CA T A G G C T

T GT A T C C G A

Transcription• After the complimentary strand is unzipped, the RNA

is written against the template strand of DNA

A CA T A G G C T

U GU A U C C G A

1 2

Transcription: step-by-step

• The enzyme RNA polymerase attaches to the DNA in the upstream (3’) region of the template strand on the promoter sequence.

• The double-stranded DNA helix unwinds• As RNA polymerase moves down the strand,

complimentary RNA bases are put down in a 5’ to 3’ direction.

• A methylated cap is added to the 5’ end of the mRNA• The transcribed portion of the helix recoils once it has

provided a template for the mRNA bases

Transcription: continued

• Once RNA polymerase reaches the terminator sequence a hairpin loop forms in the mRNA, causing it to be released

• Poly-A polymerase cleaves the end of the mRNA and synthesises a poly-A tail (adenine bases and proteins).

• A single stranded molecule called pre-messenger RNA (pre-mRNA) is produced

Post-transcription modification• The DNA in Eukaryotic genes is made up of ...– Introns (non-coding sequences)– Exons (coding sequences)

• Prokaryotic DNA does not contain introns• The entire gene is copied during transcription,

so it is necessary to the spice out the introns

Exon Exon ExonIntron Intron

Post-transcription modification

• Introns are removed by a spliceosome, which is made up of a bundle of protein factors called snerps (snRNP)

• The introns are coiled in to a shape called a lariat and released

• The remaining exons are then joined together.

One gene, multiple products

• Research reveals that a single gene is able to make a different product at different stages of development

• Also, a single gene can make one type of product in one type of tissue and a different product in another type of tissue tissue

• How is this possible?• The human genome contains approx. 25,000 genes• Yet there are approx. 120,000 recognised protein-

coding mRNA sequences.• How is this possible?

Alternative splicing of pre-mRNA

• A) Intron retention– The final product can look quite different if not all introns are spliced

out

• B) Exon juggling– Exons can be recombined in a anumber of different combinations

Translation

• The mature mRNA moves out of the nucleus, through a nuclear pore, in to the cytoplasm

• Ribosome assembles around mRNA and sequence of bases is read in blocks of 3 bases known as triplets ( = 1 codon)

• A transfer RNA (tRNA) molecule with the complementary anticodon is brought in and attaches to the mRNA

• The AUG triplet is the “start” codon

What is tRNA?• tRNA is a molecule

consisting of a single strand of 76 RNA nucleotides

• The 3 nucleotides at one end form the anticodon

• The other end forms a binding site for a specific amino acid molecule

• Amino acyl tRNA synthase catalyses the linking of each amino acid to its carrier tRNA molecule

Translation (continued)

• Each carrier molecule adds its attached amino acid to the base of the growing chain

• Not all codons code for a different amino acid• There are 64 different codons, that code for 20

amino acids• Translation continues until a “stop” codon is

reached

Codons with

corres-ponding amino acids

• The genetic code is universal

• 99.9% of species use the same triplet code for the same amino acid

Prokaryotes vs Eukaryotes• Where transcription / translation occurs

– Eukaryotes: nucleus then cyctoplasm– Prokaryotes: cytoplasm

• Speed at which it occurs– Slower in Eukaryotes due to necessity to move out to cytoplasm

as well as time required to splice mRNA

• Life span of mRNA– Prokaryotes: a few minutes– Eukaryotes: hours/days to allow time for p/t modification

• Ribosomes– Eukaryotic ribosomes are larger and have a different rRNA to

protein ratio

Gene regulation in Prokaryotes

• CASE STUDY: THE LAC OPERON• Bacteria have groups of genes that are

controlled together and are turned on/off as required

• The LAC operon is a group of genes that produce the enzymes required to preak down lactose to glucose and galactose

• The bacterium only wants to produce these enzymes when lactose is present.

The LAC Operon• Usually a repressor protein (produced by LAC

regulatory gene) sits on the controlling region• When lactose enters the cell it binds to the

repressor, and the repressor releases from the DNA

• The LAC genes will now start transcribing mRNA, which will enter a ribosome and produce the 3 enzymes required for lactose metabolism

• When concentration of lactose in the cell decreases, the lactose is released from the repressor and it returns to inhibiting the operon

Not all genes produce proteins

• Instead of mRNA, genes can also be transcribed as:– tRNA: then move out in to the cytoplasm as a transfer

molecule– rRNA: then move in to the cytoplasm to form part of a

ribosome

• The nucleolus is a region in the nucleus where rRNA is transcribed and stored until required

Mitochondrial DNA

• In Eukaryotes, mitochondrial DNA (mtDNA) is a double-stranded circular molecule

• In humans, it encompasses only 16,568 base pairs and 37 genes in total.

• Apart from the genes coding for tRNA and rRNA, the rest are involved in cellular respiration.

• Mitochondrial DNA is inherited entirely along maternal lines.