topic 4 - genetic information + variation Flashcards
what are the bases called in DNA
purine bases
adenine, guanine, thymine and cytosine
what are the bases called in RNA
pyrimidine bases
adenine, guanine, uracil, cytosine
what is the genetic code
a sequence of 3 DNA bases (triplet) codes for a specific amino acid
what is a codon
a specific triplet of bases on mRNA that codes for a specific amino acid
what is a gene
base sequence on a DNA molecule coding for a sequence of amino acids in a polypeptide chain and a functional RNA
what is a locus
location of a gene
what are introns and exons
introns- non-coding sections of DNA
exons- coding regions
comparisons between prokaryotic and eukaryotic DNA
eukaryotic is longer, linear and associated with histones
both made of DNA nucleotides containing deoxyribose, a phosphate group and nitrogenous base
nucleotides are joined together by phosphodiester bonds to form a polymer chain
3 features of genetic code
degenerate
non overlapping
universal
what is degenerate in the genetic code
there are more than one triplet of bases that codes for the same amino acid
what is universal in the genetic code
the same triplet of bases codes for the same amino acid in all organisms
why is degenerate genetic code good for mutations
if one base changes the new triplet of bases may still code for the same amino acid therefore no change will occur
what is non-overlapping in the genetic code
each base in a gene is only part of one triplet of bases that codes for one amino acid. each triplet is read as a discrete unit
why is non-overlapping genetic code good for mutations
will only affect one amino acid in sequence so minimise potential harm
what is a start codon
three bases at the start of every gene that initiate translation
what is a stop codon
three bases at the end of every gene that causes ribosomes to detach and therefore stop translation
what is the genome
an organisms complete set of genes in a cell
what is the proteome
full range of proteins that a cell is able to produce
differences between structure of mRNA and tRNA
mRNA - linear
tRNA - clover leaf shape
mRNA- codons
tRNA- anticodons
mRNA - no binding site
tRNA - amino acid binding site
steps to transcription
1) DNA helix unwinds to expose the bases to act as a template
2) only one chain acts as a template
3) unwinding of DNA strands is catalysed by DNA helicase
4) DNA helicase breaks hydrogen bonds between bases
5) free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases
6) RNA polymerase joins together the RNA nucleotides to create a new RNA polymer chain, one entire gene is copied
what happens after transcription
splicing - introns are spliced out of pre-mRNA by a protein called a spliceosome, leaving behind just the exons (the coding regions)
why do prokaryotes not undergo splicing
transcription makes mRNA as prokaryotes do not contain introns in their DNA
steps to translation
1) once modified mRNA has left the nucleus it attaches to a ribosome in the cytoplasm
2) the ribosome attaches at the start codon
3) the tRNA molecule with the complementary anti codon to the start codon will align opposite the mRNA, held in place by ribosome
4) ribosome will move along one codon on the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA
5) the 2 amino acids that have been delivered by the tRNA molecule are joined by a peptide bond, this is catalysed by an enzyme and requires ATP
6) this process continues until the ribosome reaches the stop codon at the end of the mRNA molecule, ribosome detaches and translation ends
what is a gene mutation
a change in the base sequence of the DNA
what is non dysjunction
chromosomes or chromatids do not split equally during anaphase
what is polyploidy
changes in whole sets of chromosomes occur when organisms have 3 or more sets of chromosomes rather than the usual 2 - mainly occurs in plants
how does polyploidy occur
1) each homologous pair is doubled due to DNA replication in interphase
2) non dysjunction in meiosis 1 (all chromosomes fail to separate equally) as spindle fibres have only attached to one side of chromosomes
3) in meiosis 2 there is no chromosomes in 2 gametes and 2 diploid gametes
4) if a diploid gamete fuses with a haploid gamete it will form a triploid
what is aneuploidy
changes in the number of individual chromosomes (results in gamete having one more/one less chromosome)
how does aneuploidy occur
1) non dysjunction in meiosis 1 as both spindle fibres attach to same chromosome pair
2) normal division in meiosis 2
3) makes 2 haploids with additional chromosome and 2 missing a chromosome
how does meiosis introduce variation
1) independent segregation
2) crossing over
3) random fertilisation
what is independent segregation
1) in meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell
2) it is random which side of the equator the paternal and maternal chromosomes from each homologous pair will lie
3) these pairs are separated, so one of each homologous pair ends up in the daughter cell
4) this creates a large number of possible combinations of chromosomes in the daughter cells produced
what is crossing over
1) when homologous pairs line up opposite each other at the equator and form bivalence in meiosis 1, parts of the chromatids can become twisted around each other
2) that puts tension on the chromatids causing part of them to break
3) the broken parts of the chromatid recombine with a new chromatid
4) this results in new combinations of alleles
differences between meiosis and mitosis
meiosis- 2 nuclear divisions mitosis-1
meiosis- haploid cells mitosis- diploids
meiosis- genetic variation mitosis- genetically identical
what is genetic diversity
number of different alleles of genes in a population
what is natural selection
process that leads to evolution in populations
what is evolution
change in allele frequency over many generations in a population
describe process of natural selection
1) new alleles for a gene are created by random mutations
2) if the new alleles increases the chances of the individual to survive in that environment, then they are more likely to survive and reproduce
3) this reproduction passes on the advantageous allele to next generation
4) as a result, over many generations, the new allele increases in frequency in the population
what is directional selection
-one of the extremes has the selective advantage
-occurs when there is a change in env - eg introduction of antibiotics
- antibiotic resistance increased after this
what is stabilising selection
-modal trait has selective advantage
-no change in environment
-eg human birth weights
what is a species
when 2 organisms are able to produce fertile offspring
what is courtship behaviour
a sequence of actions which is unique to each species - normally performed by males
importance of courtship
-enables them to recognise own species and opposite sex
-synchronises mating behaviour
-ensures survival of offspring as parents form a pair bond and choose a strong and healthy mate
what is a hierarchy
smaller groups arranged within larger groups
what is the species hierarchy in order
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
how do you name organisms using Binomial system
first name is genus, second name is species
what is species diversity
number of different species and individuals within each species in a community
what is genetic diversity
the variety of genes amongst all the individuals in a population of one species
what is ecosystem diversity
the range of different habitats
what is species richness
the number of different species in a community
farming techniques that reduce biodiversity
-farming hedgerows
-selective breeding
-monocultures
-overgrazing
-filling in ponds and draining wetlands
