Flashcards in Genes Deck (22)
Features of the genetic code
Degenerate - different triplets can code for the same amino acid
Non-overlapping - each base in the sequence is only part of one triplet and is only read once
Universal - each triplet codes for the same amino acid in all organisms
What are introns and exons?
Introns are non-coding parts of DNA.
Exons are coding parts of DNA.
DNA in prokaryotic and eukaryotic cells
Prokaryotic cells have DNA in the form of a single circular loop of free floating DNA. Some also have small rings of DNA called plasmids.
Eukaryotic cells have DNA folded around proteins called histones. The DNA is supercoiled and form structures called chromosomes.
Chromosomes are made up of coiled up DNA and histones and they are only visible as distinct structures when a cell is dividing. Chromosomes appear as two threads, called chromatids, joined at a single point, the centromere.
Structure of mRNA and tRNA
mRNA is a long, single strand of nucleotides.
tRNA is made up of around 80 nucleotides in a clover-like shape. The 'clover leaves' have unpaired bases sticking out of them, and one of the leaves is the anticodon loop.
DNA helicase unwinds part of the DNA
Free RNA nucleotides pair up with the exposed DNA
RNA polymerase joins the RNA nucleotides together to form pre-mRNA
The DNA re-forms and the pre-mRNA leaves the nucleus from a pore in the nuclear envelope
The pre-mRNA is spliced, removing introns so that it becomes mRNA
After leaving the nucleus, mRNA attaches to a ribosome
Free tRNA molecules with the right anticodon, and their amino acid, bind with the complementary codon on the mRNA
Peptide bonds form between the amino acids and the tRNA is released.
The second and third step repeats until a stop codon is reached
Substitution - where one nucleotide is swapped for another. Usually has a small or no impact due to the degenerate nature of the genetic code.
Deletion - where one nucleotide is lost from the sequence. Can have a huge impact due to the non-overlapping nature of the genetic code.
Individual homologous pairs of chromosomes can fail to separate in meiosis, resulting in a gamete having one more or one less chromosome. This is known as non-disjunction.
During meiosis 1, the first division, homologous chromosomes line up and twist around each other. Equivalent portions of each break off and recombine with the other chromatid; this is known as crossing over (and recombination) and is important for genetic diversity as it creates unique combinations of alleles.
In meiosis 1, the separation of homologous chromosomes into daughter cells is random; this is known as the independent segregation of homologous chromosomes and is important for genetic diversity as it creates genetically different daughter cells.
Calculating chromosome combinations
An organism can produce 2ⁿ possible different combinations of chromosomes as a result of meiosis.
Two organisms can produce (2ⁿ)² possible different combinations of chromosomes in offspring as a result of sexual reproduction.
Types of selection
Directional selection - phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.
Stabilising selection - phenotypes around the mean of the population being selected for and those at the extremes being selected against.
Definition of a species
Members of a species can produce fertile offspring.
Phylogenetic classification is a form of classification that classifies species into groups based on evolutionary relationships and shared features between organisms and their ancestors. The groups are arranged into a non-overlapping hierarchy.
Binomial naming system
The binomial system is a universal system for naming species that identifies each organism with two names. The first name is the genus it belongs to; this should be written starting with a capital letter. The second is the species it belongs to, in lower case. Names should be written in italics.
Courtship behaviour is important for the following reasons:
Recognise members of own species
Identify a mate capable of breeding
Courtship behaviour is important for species recognition as members of different species can be differentiated from one another through their different behaviour.
Species richness is a measure of the number of different species in a community.
Index of diversity
d = N(N-1) / Σn(n-1)
Agriculture and species diversity
Impacts of agriculture on species diversity:
Removal of hedgerows
Filling in ponds
Over-grazing of land
Use of pesticides and non-organic fertilisers
Conservation techniques that could be used:
Maintain existing hedgerows
Plant hedges instead of using fences
Use crop rotation that includes a nitrogen-fixing crop instead of fertilisers, and genetically modified crop instead of pesticides
Maintain existing ponds, make new ones where possible
Comparing genetic diversity
Comparison of observable characteristics - most of these are coded for by <1 gene (they are polygenic) so this method has mostly been replacing by directly observing DNA.
Comparison of DNA base sequences - this is more useful as the information can be very quickly processed by computers to see how closely related two species are.
Comparison of mRNA base sequences - this is also useful and is very similar to comparing DNA.
Comparison of amino acid sequences in proteins - this can be used to see how closely related each species are by writing down a number of amino acids in order and counting the number of similarities/differences.