Topic 4 - Genetics Flashcards
What is a gene?
Section of DNA that codes for making a polypeptide and functional RNA
Define Locus
Location of a particular gene on a chromosome
Define allele
Alternative form of a gene
Describe how DNA is stored in eukaryotes
stored as chromosomes in the nucleus DNA is tightly wound around proteins called histones
Chromosomes are linear in shape
Describe how DNA is stored in prokaryotes
DNA is shorter and circular
dna isn’t wound around histones
Define homologous pairs
Pairs of matching chromosomes
the chromsomes are exactly the same size, have exactly the same genes but might have different alleles
Define transcription
Where one gene on the DNA is copied into mRNA
Define translation
Where the mRNA joins with a ribosome, and corresponding tRNA molecules brings the specific amino acid the codon codes for
Give the first stage of transcription
DNA helix unwinds - DNA helices breaks hydrogen bonds - one chain of the DNA acts as a template
Give the second stage of transcription
Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases
Give the third stage of transcription
RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied
What happens to pre-mRNA after transcription?
Pre-mRNA has to be modified to become mRNA that is ready to leave the nucleus and take part in translation.
The introns need to be spliced out
What are introns?
Introns don’t code amino acids
Describe what happens to the introns after transcription
The introns are spliced out by a protein (splicesome). This just leaves the exons (coding regions)
Describe the first stage of translation
Modified mRNA leaves the nucleus and attaches to a ribosome. Ribosome attaches at start codon. tRNA molecule with complementary anticodon to the start codon aligns opposite mRNA, held in place by the ribosome.
Describe the second stage of translation
Ribosome will move along the mRNA molecule to allow other complementary tRNA to attach to the next codon on the mRNA.
The two 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.
Describe the third stage of translation
This continues until the ribosome reaches the stop codon at the end of the mRNA molecule. The stop codon doesn’t code for an amino acid and therefore the ribosome detaches and translation ends.
describe the DNA found in chloroplasts and mitochondria
similar to prokaryotic DNA, in that it’s short and circular
what is a start codon
they are at the start of every gene and they code for an amino acid
what is a stop codon
occur at the end of every gene that do not code for an amino acid
what is meant by the genetic code being degenerate
64 combinations is more than is needed to code for 20 amino acids
each amino acid is coded by more than 1 triplet of bases
what is meant by the genetic code being universal
the same triplet of bases codes for the same amino acids in all organisms
what is meant by the genetic code being non-overlapping
each base in a gene is only part of one triplet of bases that codes for one amino acid
describe meiosis
produces 4 daughter cells that are genetically different
4 haploid daughter cells
2 nuclear divisions
haploid cell
introduces genteic variation
describe the process of crossing over (4 marks)
in meiosis I, parts of the chromatid can become twisted around each other
this puts tension on the chromatids, causing parts of the chromatid to break
the broken parts of the chromatid recombine with the another chromatid
results in new combinations of alleles
describe independent segregation (3 marks)
in meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell
it’s random which side of the equator the paternal and maternal chromosomes from each homologous pair lie
these pairs are separated, so one of each homologous pair ends up in the daughter cell
give the definition of genetic diversity
the number of different alleles of genes in a population
when can natural selection occur
only occur when there is genetic diversity within the population
define evolution
the change in allele frequency over many generations in a population
what does natural selection result in
species becoming better adapted to their environment, adaptations may be anatomical, physiological or behavioural
describe the process of natural selection (4 marks)
new alleles for a gene are created by random mutations
if the new alleles increases the chances of the individual to survive in that environment, then they are more likely to survive and reproduce
this reproduction passes on the advantageous allele to the next generation
over many generations the new allele increases in frequency in the population
why is courtship essential
essential for successful mating and species recognition
define species
a group of similar organism that can breed to make fertile offspring
what are courtship rituals
sequence of actions which is unique to a species
most courtship rituals are performed by males
they can include a sequence of dance moves, sounds, colourful feathers
why is courtship important to ensure successful reproduction?
enables them to recognise own species and opposite sex
synchronises mating behaviour - indicates sexually mature and in season (releasing egg)
why is courtship important to ensure the survival of the offspring?
form a pair bond
choose a strong and healthy mate
give the two types of selection
directional selection
stabilising selection
describe directional selection and give an example
one of the extremes has the selective advantage
occurs when there is a change in the environment
the modal trait changes
e.g. antibiotic resistance in bacteria
describe stabilising selection and give an example
the modal trait has the selective advantage
occurs when there is no change in the environment
modal trait remains the same
standard deviation decreases, as individuals with the extreme trait decrease
e.g. human birth weights
when can chromosome mutations arise
can arise spontaneously by chromosome non-disjunction during meiosis
define non-disjunction
when the chromosomes or chromatids do not split equally during anaphase
describe polyploidy
changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual 2
describe aneuploidy (3 marks)
changes in the number of individual chromosomes
sometimes individual homologous pairs of chromosomes fail to separate during meiosis
this is non-disjunction and usually results in a gamete having one more or one fewer chromosome
