genes Flashcards
How do eukaryotic cells store DNA?
Contain linear DNA that exists as chromosomes. DNA molecule is really long, so wound around proteins called histones, which also help to support the DNA. DNA then coiled very tightly to form chromosomes. Mitochondria and chloroplasts have their own DNA which are pretty similar to prokaryotic DNA
How do prokaryotic cells store their DNA?
Also carry DNA as chromosomes but the DNA molecules are shorter and circular. DNA isn’t wound around histones but condenses by supercoiling to fit into the cell
What are chromosomes?
Thread-like structures, each made up of one long molecule of DNA and its associated proteins
Define gene:
Sequence of DNA bases that codes for either a polypeptide or functional RNA
Define codon:
Sequence of 3 bases in a gene that codes for an amino acid
Define genome:
Complete set of genes in a cell
Define proteome:
Complete set of proteins that the cell is able to produce
What are alleles?
Different forms of the same gene, order of bases in each allele is slightly different so they code for different versions of the same polypeptide
What is functional RNA?
RNA other than mRNA, which perform special tasks during protein synthesis (e.g. tRNA or rRNA)
What are introns and exons?
Introns- sections of genes which code for polypeptides that don’t code for amino acids
Exons- all the sections of a gene that do code for an amino acid
What are non-coding multiple repeats?
DNA sequences that repeat over and over that don’t code for amino acids
What are homologous chromosomes?
Pairs of matching chromosomes that are the same size and have the same genes, although could have different alleles.
What is protein synthesis?
Production of proteins from the info contained within a cell’s DNA
What is transcription?
Where the DNA is copied into mRNA
What is translation?
Where the mRNA joins with a ribosome and the code it carries is used to synthesise a protein
What is the role and structure of messenger RNA?
Made during transcription. Carries the genetic code from the DNA to the ribosomes, where it is used to make a protein during translation. mRNA is a single polynucleotide strand
What is the role and structure of transfer RNA?
Involved in translation. Carries the amino acid that are used to make proteins to the ribosomes. tRNA is a single polynucleotide that has been folded into a clover shape. H bonds between the specific base pairs hold molecule in this shape. Every tRNA molecule has a specific sequence of three bases at one end called an anticodon. Also has an amino acid binding site at other end.
Outline the process of transcription:
RNA polymerase attaches to the DNA: RNA polymerase attaches to the double helix at the start of a gene. The hydrogen bonds between the two DNA strands in the gene break (by DNA helicase), separating the starnds and exposing some bases. ONe strand then used to make an mRNA copy.
Complementary mRNA is formed: RNA polymerase lines up free RNA nucleotides alongside the exposed bases on template, attracted due to complementary base pairing. mRNA strand forms a complementary copy of DNA. Nucleotides joined by RNA polymerase to form an mRNA molecule
RNA polymerase moves down the DNA strand: Moves along the DNA, assembling the mRNA. Hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by, and strands coil back into double helix
RNA polymerase reaches stop signal: stops making mRNA and detaches from the DNA. In eukaryotes mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosomes in the cytoplasm.
Outline the process of translation:
mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it. ATP provides the energy needed for the bond between the amino acid and tRNA to form. A tRNA molecule with an anticodon thats complementary to the first codon on the mRNA attaches itself to the mRNA by complementary base pairing. A second tRNA attaches itself to the next codon on the mRNA in the same way. Two amino acids attached to the tRNA molecules join by peptide bond. First tRNA molecule moves away , leaving its amino acid behind. Another tRNA molecule joins in the same way. Ribosome moves along the mRNA strand. Process continues producing a chain of linked amino acids until there’s a stop signal on the mRNA, so polypeptide moves away from ribosome and translation is complete
What is pre-mRNA?
mRNA strands containing introns and exons
What is splicing?
Introns are removed and exons are joined forming mRNA
What is the genetic code?
Sequence of base triplets in mRNA which code for specific amino acids
Describe the three qualities of the genetic code:
Non-overlapping: base triplets don’t share their bases
Degenerate: more possible combinations of triplets than amino acid
Universal: the same specific base triplets code for the same amino acids in all living things
Define diploid:
Each cell contains two of each chromosomes, one from the mum and one from the dad
Human diploid number is 46
Define haploid:
Only contain one copy of each chromosome in a homologous pair
Human haploid number is 23
Explain fertilisation:
A haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes. During sexual reproduction, any sperm can fertilise any egg, which produces zygotes with different combos of chromosomes to both parents– this increases genetic diversity within a species
Outline meiosis:
- Before meiosis starts, the DNA unravels and replicates so there are two copies of each chromosome- chromatids
- DNA condenses to form double armed chromosomes each made from two sister chromatids joined by a centromere.
- Meiosis I- the chromosomes arrange themselves into homologous pairs
- These homologous pairs are then separated, halving the chromosome number
- Meiosis II- pairs of sister chromatids are separated (centromere divides)
- Four genetically different haploid cells are produces
What is crossing over of chromosomes?
During meiosis I, chromatids twist around each other and parts of chromatids swap over. The chromatids still contain the same genes but now have a different combo of alleles.
What is independent segregation of chromosomes?
When the homologous pairs are separated in meiosis I, it is completely random which chromosome from each pair ends up in the daughter cell, so the 4 daughter cells have completely different combos of maternal and paternal chromosomes. This ‘shuffling’ of chromosomes leads to genetic variation in any potential offspring.
What is a mutation?
A change in the DNA base sequence of chromosomes.
What are substitution and deletion mutations?
Substitution: one base is substituted with another
Deletion: one base is deleted
What is a mutagenic agent?
Increases the probability of a mutation occurring
UV and ionising radiation, carcinogenic chemicals and viruses.
What is chromosome non-disjunction?
Failure of chromosomes to separate properly
What is the effect of gene mutations?
Degenerate nature means that some amino acids are coded for by more than one DNA triplet, so not all substitutions will result in a change to the amino acid sequence of a protein. Deletion will always lead to changes in the amino acid sequence, and causes a shift in all the base triplets after it
What is a chromosome mutation?
If meiosis goes wrong, the cells produced contain variations in the number of whole chromosomes or parts of chromosomes. Chromosome mutation leads to inherited conditions because the errors are present in the gametes.
What is genetic diversity?
The number of different alleles of genes in a species/populations
Why is genetic diversity important?
If a population has low genetic diversity it might not be able to adapt to a change in the environment and the whole population could be wiped out by a single event
What increases genetic diversity within a population?
Mutations in the DNA forming new alleles (could be advantageous or lead to problems)
Different alleles being introduced into a population when individuals from another population migrate into it and reproduce (gene flow)
What is a genetic bottleneck?
An event that causes a big reduction in a population. This reduces the number of different alleles in the gene pool and so reduces genetic diversity. Survivors reproduce and a larger population is created from a few individuals.
What is the Founder Effect?
Describes what happens when just a few organisms from a population start a new colony and there are only a small number of different alleles in the initial gene pool. The frequency of each allele in the new colony might be very different to the frequency of those in the original population- this may lead to a higher incidence of genetic disease
Define natural selection:
When the allele codes for a characteristic that increases the chances of an organism surviving, its frequency within the population can increase
Define evolution:
The gradual change in a species over time. It has led to the huge diversity of living organisms on earth
Define adaptations:
Feature that helps organisms to survive their environment
Describe the process of natural selection:
- Individuals that have an allele that increase their chance of survival are more likely to survive and reproduce and pass on their genes than individuals with less advantageous alleles
- This means a greater proportion of the next generation inherits the beneficial allele
- They, in turn, are more likely to survive, reproduce and pass on their genes to their offspring
- So the frequency of the beneficial allele in the population increases from generation to generation
- Over generations, this leads to evolution as the beneficial allele in the population becomes more common in the population
What are the three types of adaptations?
Behavioural- possums play dead
Physiological- hibernation
Anatomical (structural)- blubber
What is directional selection?
Where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce. This could be in response to an environmental change
What is stabilising selection?
Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce. It occurs when the environment isn’t changing and it reduces the range of possible characteristics
What is phylogeny?
Study of the evolutionary history of groups of organsisms
What is taxonomy?
The science of classification
Name the 8 taxas in order:
Domain, kingdom, phylum, class, order, family, genus and species
What is the binomial system?
Genus and species
What is courtship behaviour?
Carried out by organisms to attract a mate of the right species
Why is courtship important?
Ensures successful reproduction, as can recognise species and sex and syncronizes mating behaviour
Ensures survival of the offspring, as forms a pair bond, and allows them to choose a strong healthy mate
How can courtship behaviour be used to classify a species?
Courtship behaviour is species specific
The more similar courtship behaviour, the more closely related species are
How can we use genome sequencing to classify species?
Entire base sequence of an organisms DNA can be determined. Sequence of one organism can then be compared to the DNA base sequence of another organism to see how closely related they are.
How can we use amino acid sequences to classify species?
Sequence of amino acids in a protein is coded for by the base sequence in DNA. Related organisms have similar base sequences and so similar amino acid sequences in their protein
How can we use immunological comparisons to classify species?
Similar proteins will also bind to the same antibodies
Define variation and the two different types:
Differences that exist between individuals
Interspecific- between species
Intraspecific- within species
Define biodiversity, habitat and community:
Biodiversity- variety of organisms living in an area
Habitat- place where organisms live
Community- all the populations of different species in a habitat
What is species richness?
Measure of the number of different species in a community
How do you calculate the index of diversity?
d= N(N-1) ÷ Σn(n-1)
What is variation caused by?
Genetic factors- different genes between different species, different alleles within a species
Environmental factors
What is the impact of agriculture on biodiversity?
Woodland clearance and hedgerow removal- destroys habitats and food sources so species die or are forced to migrate
Pesticides- kills pest and the food source for species that feed on pests
Herbicides- reduces plant diversity and destroys food source
Monoculture- fields contain only one type of plant so support fewer organisms
