Genes: organization, function and evolution

WHAT IS A GENE?

• DNA encoding for a trait

• What is DNA?

• Nitgogenous bases + sugar phosphate

“backbone”

• Organised in double helix

• Bases: Adenine (A), Guanine (G), Cytosine, Thymine (T)

• DNA has two extremely important properties:

1. It contains the instructions for how organisms should be put together in the enormous variety of ways that characterizes life on earth.

2. It can be copied or replicated so that these instructions are passed on to successive generations.

• As evolutionist we are interested in another property of DNA: that the process of replication is not perfect so that errors, or mutations, continually arise which provide the raw material upon which evolution works.

• Furthermore, because mutations gradually accumulate through time, so that species which have diverged recently usually differ by fewer mutations than those which went their separate ways in the distant past, we can use the sequence of mutation to retrace the pathway of evolutionary change.

• Genes are therefore valuable archives of evolutionary information.

• Types of mutations:

• Most involve in the replacement, or substitution, of one base for another in the DNA sequence.

• Point mutations can be placed into two categories:

• transitions – a purine nucleotide (A and G) is substituted to another

purine or a pyrimide (C and T) is replaced by another pyrimidine

• transversions – a pyrimidine is substitute for a purine, or vice versa

• Another class of mutations are those that lead to insertions and deletions of nucleotides (indels)

THE STRUCTURE OF GENES

• Although genes are composed of DNA, not all DNA forms genes. This is especially true of eukaryotes where most DNA appears to be non-coding DNA and may have no function.

• In contrast, coding DNA produces proteins.

* The genetic code

GENOME ORGANIZATION

• Not all DNA in plants

is stored in the cell nucleus:

• Nuclear

• Mitochondrial

• Chloroplast

• Mitochondrial genome

• Plants and animals (most eukaryotes)

• Circular

• Size: 6 kb (kilobases) – 2000 kb

• Maternally inherited

• Helps degrade fats and sugars, including those important in respiration.

• The mt genome rearranges itself frequently, so many rearranged forms can occur in the same cell. This means that rearrangements of the genome occur so often within individual plants that they do not characterize or differentiate species or groups of species are thus not especially useful for inferring relationships.

• Chloroplast genome

• Plants and algae

• Circular

• Size: 120-220 kb

• Maternally inherited

• The most obvious feature: LSC, SSC, IR (two regions that encode the same genes, but in opposite directions)

• Rearrangements of the cp genome are rare enough in evolution that they can be used to define major groups.

• Evolve more slowly than the nuclear gene but faster than the plant mtDNA genes

• Nuclear genome

• Plants, algae and animals, human

• Linear

• Size: huge!!! From 60 Mbp – 150,000 Mbp

• Biparentally inherited

• Nuclear genome comprises the chromosomes of organism. Each chromosome consists of one long, double stranded DNA molecule with many genes located along its length.

• It tends to be highly repetitive, meaning there are many segments that are repeated many times throughout the chromosomes.

• It evolves faster than

the cpDNA and mtDNA