11 Gene function:genes in action

Sea in the blood

• Various kinds of haemoglobin are found in red blood cells.

• Each kind of haemoglobin consists of four protein chains each with an iron-containing haem molecule.

• The gene that controls the production of the beta chains of haemoglobin A is the HBB gene on the number-11 chromosome.

• Absence of beta chains is an inherited disorder known as beta thalassaemia.

Genes in action• Coded genetic instructions are located in the DNA of the

nucleus of eukaryote organisms.• DNA and RNA are both nucleic acids, but differ in several

ways.• During transcription, the information in the template strand

of the• DNA of a gene is copied into a RNA molecule.• The base sequence in a single strand of DNA acts as a

template to guide formation of pre-mRNA.• The final mRNA molecule results when regions corresponding

to introns are removed.

transcription

splicing

From DNA to mRNA: step by step

Translation: decoding geneticinstructions

• DNA in the nucleus = master plan with complete set of instructions

• mRNA = working copy of one instruction• ribosomes = construction site• tRNA = carriers of raw material• amino acids = raw material• protein chain = end product

tRNA- up closeSimplified

The codons for all amino acids

transcription and translation

Comparing prokaryotes andeukaryotes

Prokaryotic DNA Eukaryotic DNA

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

Each chromosome is a linear molecule of DNA.

DNA is naked (see figure 11.17). DNA exists in complexes with proteins called histones (refer back to figure 1.30).

DNA comprises unique nucleotide sequences.

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 (exons) of genes are interrupted by introns.

As well as the main chromosome, additional DNA in the form of plasmids is present.

Plasmids are absent.

• Gene translation occurs in the cytoplasm and involves cell organelles known as ribosomes.

• mRNA instructions are encoded as sets of three non-overlapping bases called codons.

• Translation commences when a START codon is translated; this codon not only starts translation but also puts the amino acid, met, in place.

• As each mRNA codon is translated, a specific amino acid is brought into place by the tRNA molecule with the complementary anti-codon.

• Translation ceases when a STOP codon is reached.

• The end product of translation is a protein chain or polypeptide.

• Differences exist between the organisation of the genomes of prokaryotes and of eukaryotes.

• Gene action in these two groups is essentially similar but with some minor differences.

How many alleles?• In Mendelian genetics, genes are often identified as having

a few, often just two, alleles. This is a simplification because one gene can have a very large number of different forms or alleles. Every alteration of one gene is an allele of that gene.

• Alterations of a gene can include many kinds of changes, such as:

base substitutions base additions (insertions) base deletionsand these changes may involve one or more bases.

We commonly recognise just two different alleles of a gene in terms of phenotype, such as ‘unaffected’ or ‘showing a disorder’.

• In beta-zero thalassaemia, no beta protein chains are produced.

• Beta-zero thalassaemia can result from a single base substitution in the coding sequence of the HBB gene that affects both the mRNA transcribed and the protein made during translation.

• Many mutations of a gene can occur so that many alleles of one gene can exist.

All genes produce RNA . . . mostthen produce protein

• All active genes produce RNAs of some kind.• Most genes transcribe mRNAs that are then

translated into proteins.• Some genes produce other kinds of RNA, such

as tRNA and rRNA, and these are the end products.

• Genes can be classified in various ways.Structural, regulator, homeolytic

Which model of DNA replication?

Self-replication: copying itself• DNA molecules can undergo self-replication.• DNA replication requires an existing DNA molecule to act

as a template for the manufacture of new complementary strands.

• Nucleotides are the raw materials from which the new strands are constructed.

• Several enzymes are involved in the process of DNA replication.

• DNA replication occurs during interphase in body cells that can reproduce by mitosis, and during meiosis in germline cells in the gonads as part of gamete formation.

When and where are genes active?• Genes differ in the period over which they are active

or being expressed.• Some genes act in all living cells, while other genes

are active in certain cells only.• Microarray technology provides a means of identifying

all of the active genes in various cell populations.• RNA interference (RNAi) provides a means of

selectively targeting and silencing genes.• Small interfering RNAs (siRNAs) produced in cells are

the active molecules in gene silencing.

Case study: Blue roses at last!