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Describe DNA in eukaryotic cells
Nucleus contains long, linear DNA molecules with associated proteins as chromosomes, DNA is wound around histones (proteins) for strength and so it can fit, coiled very tightly to make compact chromosomes
Describe DNA in mitochondria and chloroplasts
Have their own DNA, circular and shorter and not associated with proteins
Describe DNA in prokaryotic cells
Carry DNA as chromosomes, shorter, circular, not associated with proteins, condenses to fit in cell by supercoiling
What is a gene?
A base sequence of DNA that codes for: The amino acid sequence of a polypeptide A functional RNA
How many bases make up a triplet?
3
How many different amino acids there are?
20
How many combinations of amino acids are there?
64
Name and describe 3 features of the genetic code
Universal - same bases for all species Non-overlapping - read in triplets Degenerate - different triplet codes for the same amino acid
What is functional RNA and what does it do?
RNA molecules other than mRNA (e.g. tRNA and rRNA) which perform special tasks during protein synthesis
What is non-coding DNA?
Introns - removed by splicing Exons - do code for amino acids
Define alleles
Different versions of the same gene
What are homologous chromosomes?
Pairs of matching chromosomes, same size and same genes, but could have different alleles
What is the locus?
The fixed position of a gene on each chromosome in a homologous pair
What is the genome?
Complete set of genes in a cell
What is the proteome?
Full range of proteins a cell is able to produce
Describe RNA
Single polynucleotide strand, contains uracil instead of thymine
What is the role of messenger RNA (mRNA)?
Made during transcription, carries genetic code from DNA to ribosomes, where it is used to make a protein during translation, single polynucleotide strand, groups of 3 adjacent bases = codons
What is the role of transfer RNA (tRNA)?
Involved in translation, carries amino acids used to make proteins to ribosomes, single polynucleotide folded, H-bonds hold it in shape, have specific anticodon and amino acid binding site
Where does transcription occur in eukaryotes?
Nucleus
Where does transcription occur in prokaryotes?
Ribosomes in cytoplasm, directly produces mRNA from DNA (no splicing occurs)
Describe the process of transcription
RNA polymerase attaches to DNA at beginning of gene, breaks H-bonds, separating strands
RNA polymerase lines up free nucleotides, complementary mRNA formed, T is replaced with U, RNA polymerase joins together free bases, forming an mRNA strand
RNA polymerase moves down DNA, mRNA assembles, H-bonds reform
RNA polymerase reaches stop signal and detaches
In eukaryotes, splicing occurs, introns are removed
Where does translation take place in prokaryotes and eukaryotes?
Ribosomes in cytoplasm
Describe the process of translation
mRNA attaches to ribosome and tRNA molecules carry amino acids to it, ATP provides energy for bond between amino acid and tRNA to form
tRNA (carrying amino acid) with complementary anticodon attaches to mRNA, a second tRNA does the same
The 2 amino acids join by a peptide bond
Process continues as ribosome moves along mRNA, produces polypeptide chain, until it reaches a stop signal
Polypeptide chain is complete
Normal body cells have a ________ number
Diploid (2n)
Gametes have a _________ number
Haploid (n)
What is the human haploid and diploid number?
Diploid = 46 Haploid = 23
What happens during sexual reproduction?
2 gametes joining together at fertilisation to form a zygote, which divides and develops into a new organism
What happens during fertilisation?
Haploid egg fuses with haploid sperm, to make a cell with normal diploid number of chromosomes (1/2 from father, 1/2 from mother)
Describe the process of meiosis
Before meiosis starts, DNA unravels and replicates, so there are 2 copies chromosome = sister chromatids
DNA condenses to form double-armed chromosomes, each made from 2 sister chromatids joined by centromeres
First division - chromosomes arrange themselves in homologous pairs then homologous pairs separated, halving the chromosome number
Second division - Pair of sister chromatids are separated (centromere divides)
4 haploid cells that are genetically different from each other are produced
Where does meiosis occur?
In reproductive organs of multicellular, eukaryotic organisms
What is a mutation?
Change in DNA base sequence of chromosomes
What is base addition?
One or more bases added, changes whole code
What is base deletion?
One or more bases deleted, changes whole code
What is base substitution?
One base is substituted for another, only 1 triplet will be changed, sometimes changes degenerate code
What do mutagenic agents do?
Increase probability of a mutation occurring
Name 4 examples of mutagenic agents
UV radiation, ionising radiation, some chemicals, some viruses
What do chromosome mutations lead to?
When meiosis goes wrong, cells produced contain variations in numbers of whole or parts of chromosomes, leads to inherited conditions
What happens during non-disjuction in meiosis 1?
Homologous pairs fail to separate
What happens during non-disjunction in meiosis 2?
Zygote haas 3 copies of a chromosome
Describe crossing over of chromatids
During meiosis 1, homologous chromosomes pair up
Chromatids twist and entangle (form chiasma) and swap over same length of genes
New combinations of alleles formed
Describe independent segregation of chromosomes
When homologous pairs are separated in meiosis 1, it is completely random which chromosome from each pair ends up in which daughter cell, so the 4 daughter cells have different combinations of alleles, leads to genetic variation
What is variation?
Difference between same species organisms
Define genetic diversity
Total number of different alleles of genes in a population
Define population
Total number of organisms in a habitat
Define allele frequency
Frequency of specific alleles in a population
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
How is genetic diversity within a population increased?
Mutations in DNA forming new alleles, gene flow (different alleles being introduced into a population when individuals from another population migrate into it and reproduce)
What are genetic bottlenecks?
An event that causes a big reduction in a population, reduces number of different alleles in a gene pool which reduces genetic diversity, the survivors reproduce and a larger population is created from a few individuals
What is the founder effect?
When 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, genetic diversity is reduced
Why is natural selection good?
It increases proportions of advantageous alleles within a population
Describe the process of natural selection
Presence of advantageous alleles
Survival of the fittest
More likely to survive, reproduce and pass on genes to offspring
Frequency of allele increases
Leads to evolution
What is evolution?
The gradual change in species over time, leads to diversity of living organisms on earth
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, occurs when environment isn't changing and it reduces the range of possible characteristics
What is phylogeny?
The study of evolutionary history of groups of organisms
What do phylogenic trees show? Draw an example
Closely related species diverged away from each other more recently
What is taxonomy?
The science of classification, naming organisms and putting them into groups
What is speciation?
The production of a new species
What does phylogenetic classification show?
Organises species on how closely they are related
Name the groups of classification
Domain Kingdom Phylum Class Order Family Genus Species
As you go down the groups of classification, what happens?
More groups, but fewer organisms in each group, organisms in each group become more closely related
Why are classification systems constantly being updated?
Discoveries of new species and new evidence about known organisms (e.g. DNA sequencing data)
What taxons does the binomial naming system use? And why is this system used?
Genus and species, universal
How do you write out a binomial name properly?
First - genus, second - species, in italics or underlined, 1st letter - uppercase, 1st letter of second word - lowercase
What is courtship behaviour?
Behaviour carried out by organisms to attract a mate of the same species
Name 2 simple courtship behaviours
Releasing a chemical using sound, visual displays
Name 2 complex courtship behaviours
Dancing, building
Who carries out courtship behaviours?
Male or female, may sometimes involve both sexes
How can courtship behaviour be used to classify species?
Courtship behaviour is species specific, the more closely related a species is, the more similar their courtship behaviour
Define biodiversity
Number of different ecosystems and communities of organisms, number of different species and degree of genetic diversity within those species
Define habitat
Place where an organism lives
Define community
All the different species that live together in a habitat
What is species richness?
A measure of the number of different species in a community
What is the index of diversity? And how is it calculated?
Relationship between number of species in a community and number of individuals in a species
How is agriculture threatening biodiversity?
Woodland clearance, hedgerow removal, pesticides, herbicides, monoculture
How can diversity be investigated?
Frequency of measurable or observable characteristics
Base sequence of DNA / immunology
Base sequence of mRNA
Amino acid sequence of proteins encoded by DNA and mRNA
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