Gene function: genes in actionChapter 11

Genes in action

• When genes are active, their instructions are decoded and are expressed in the phenotype

• Gene action involves two processes: – TRANSCRIPTION– TRANSLATION

Eukaryotic cell

Prokaryotic cell

3‘

3‘5‘

5‘DNA strand (template)

TRANSCRIPTION

TRANSLATION

A C C A A A C C G A G T

U G G U U U G G C U C A

Trp Phe Gly Ser

mRNA

Protein

Stages of transcription

Post-translation modification: introns are cut out of pre-mRNA -> mRNA

mRNA is chemically capped and a ploy-A tail is added so it can moved across nuclear membrane

RNA splicing

Alternative RNA splicing

Translation• the 'language' of base triplets is translated

into the 'language' of amino acids• tRNA 'speaks' both languages• mRNA is read in a 5' - 3' directiontRNA– short single strand of nucleotides (approx.

80)– 3 leaf clover shape– If an amino acid is attached it is charged– 2 specific binding sites (amino acid attachment site & anticodon

site)– Aminoacyl tRNA synthase catalyses the linking of aa's to its

particular tRNA carrier– Anticodon site has 3 bases that are complementary to a triplet

codon on the mRNA

Comparing prokaryotes & eukaryotes Prokaryotic DNA Eukaryotic DNA

The main chromosome is a circular molecule of DNA, called plasmid

Each chromosome is a linear molecule of DNA

DNA is naked DNA exists in complexes with proteins called histones

DNA comprises unique nucleotide sequence DNA contains many repeated nucleotide sequences

DNA is free within the cell DNA is enclosed within the nucleus

Coding sequences of genes are uninterrupted

Coding sequences of genes are interrupted by non-coding sequences

Additional DNA in form of plasmid present No plasmid

• Location of gene action: E: Transcritption, Translation in different cell compartment, P: everything in Cytoplasm

• mRNA: P: lasts few min, E: lasts for hours or days• Ribosomes: differ in size for P & E, the difference can be tested with

antibiotics

Thalassaemia

• What is it?• What are the symptoms?• How is it caused?• What treatment is there?• How do the various types differ?

What is it?• Red blood cells are smaller and don't contain as much haemoglobin (so RBCs die

faster)Haemoglobin has 4 protein chains and haem molecules (contain iron)

What are the symptoms?• Anaemia

How is it caused?• recessive allele (HBB Gene) Chromosome 11• single base mutation --> nonsense mutation (stop codon) -> shorter aa chain (Beta

chain) in the haemoglobin

What treatment is there?• Blood transfusion (add haem) • Desferal - gathers/binds to the unwanted iron form around organs to help with

excretion.

How do the various types differ?• Beta - change in the upstream region of the genes coding region (changes the TATA

Promoter region)

Repetition: Location of genes (p.307)

• The position occupied by a gene on a chromosome is known as it's locus (pl = loci)1st number / letter = chromosome letter p (short arm) or q (long arm) last number = region of the chromosome arm

• eg: DMD gene controls the production of the muscle protein, dystrophin

Location: Xp2

• HBB location = 11p1• CFTR location = 7q3

Some genes produce another intermediary other than mRNA (other kinds of RNA)

Ribosomal RNA (rRNA):• produced in large quantities in the nucleus• stored in nucleus forming the nucleolus• when needed to form part of the structure of

ribosomes, it moves into the cytoplasm• genes on the short arms of chromosomes

13,14,15,21 & 22 code for rRNA production= nucleolar organiser regions (NORs) (seen as secondary narrowing)

Structural and regulator genes• Structural genes that code for proteins that are part of the structure &

functioning of an organism (eg. Enzymes)• Regulator genes code for proteins that control the actions of other genes

– DNA-binding proteins: bind to regions of nuclear DNA & directly switch other genes on/off

– Some proteins bind to receptors on the membrane of cells and trigger a series of intracellular reactions that switch genes on/off

– Master genes = homeotic genes (regulator genes)• Important in embryonic development

ie: make sure that flies have antennae on their heads and not extra legs!

• Mammals homeotic genes are known as HOX genes• Important just like in insects - arms & legs in the right place, ribs on some

vertebrae and not others etc...• Arranged in 4 gene clusters on 4 different chromosomes (7, 17, 12, 2)

DNA replication• DNA is unwinded (unzipped) by helicase;

RNA polymerase (enzyme) attaches to DNA strand

• Leading Strand – 3` (copied in order)• Lagging strand – 5` (more complex -

copied in segments (Okazaki fragments))• Copied from 5' to 3' (so new nucleotides

are added heading towards the 3')• Replication fork (Y-junction) - multiple

enzymes involvedOkazaki Fragments: short segments of DNA

made when replicating the lagging strand

DNA polymerase: helps complementary base pairs to join with the leading strand

Conservative vs Semi-conservative Model of DNA Replication

• By allowing Bacteria to grow in heavy Nitrogen and then to allow for production of one new generation in normal Nitrogen, the old DNA and the new DNA (products of DNA replication) should be split by centrifugation

• They're not, so DNA replication is Semi-conservative!

• Draw a diagram to represent this

Gene Activity• Some genes actively make mRNA and proteins only during a short

period of the life span• eg: Huntington disease has a late onset (after the age of 40) so the

dominant H allele is not expressed on the phenotype until well into adulthood

• Others are active throughout a persons life• eg: genes responsible for the production of enzymes involved in

respiration

• Some genes are active in every cell• Others only act on particular cells• eg: the DMD gene for protein dystrophin (muscular dystrophy) is only

active in skeletal muscle cells

Microarrays

• DNA arrays / gene chips now allows the study of large numbers of genes to:– identify which genes are active / switched off– compare gene expression in different cells– compare active genes in same cells under different

conditions

Switching genes off• Parkinson's disease is apparently caused by the overproduction of a

particular protein• Imagine if gene technology could help to switch off a mutant gene.

• RNA interference (RNAi) used to silence or turn down genes (by using dsRNA)

• Does not act directly on the DNA but breaks down the mRNA produced by one specific gene

• Long dsRNA is cut by a Dicer so this RNA is in short fragments called small interfering RNA (siRNA). siRNA combines with particular cellular proteins to form an RNA induced silencing complex (RISC)

• Draw a diagram to represent this (use p. 410 to help).