3.4 Genetic information, variation and relationships between organisms Flashcards
Gene
- a section of DNA that contains a code for making a polypeptide and functional RNA
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Locus
- the location of a particular gene on a chromosome
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Allele
- a different version of the same gene
Chromosome
- threadlike structure
- composed of tightly coiled DNA wrapped around histones (if it is a eukaryotic cell)
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Homologous chromosome
- a pair of chromosomes that have the same genes
- ∴ they are the same size
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Eukaryotic DNA
- DNA is stored as chromosomes inside the nucleus
- linear chromosomes
- DNA is tightly coiled and wrapped around proteins called histones
Prokaryotic DNA
- DNA molecules are shorter and circular
- DNA is not wound around histones. Instead, it supercoils to fit in the cell.
Codon
- 3 bases on mRNA that code for an amino acid
Start codon
- 3 bases at the start of an mRNA sequence which help to initiate translation
Stop codon
- 3 bases at the end of every gene that do not code for an amino acid
- this causes ribosomes to detach
- and ∴ stops translation
Genetic code
- an amino acid is coded for by 3 DNA bases which are described as the “triplet code”
What is meant by ‘the genetic code is degenerate’?
- each amino acid is coded for by
more than one triplet of bases
What is the advantage of the genetic code being degenerate?
- if a substitution mutation occurs, the new triplet of bases may still code for the same amino acid
- ∴ the mutation will have no impact on the final protein produced
What is the advantage of the genetic code being universal?
- genetic engineering is possible
- a human gene can be inserted into another organism
- e.g. human gene for insulin inserted into bacteria to make insulin
What is the advantage of the genetic code being non-overlapping?
- if a point mutation occurs, it will only affect one codon and ∴ one amino acid
Triplet code
- an amino acid is coded for by 3 bases
Mutation
- a change in the DNA
- can be a gene or chromosome mutation
What is meant by ‘the genetic code is universal’?
- the same triplet of bases codes for the same amino acid in all organisms
What is meant by ‘the genetic code is non-overlapping’?
- each base in a gene is only part of one triplet of bases that codes for one amino acid
- ∴ each codon is read as a discrete unit
Introns
- non-coding sequence of DNA
Exons
- sequences of DNA that code for amino acids
What is splicing?
- post-transcription modification
- removing introns
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Genome
- the complete set of genes in a cell
Proteome
- The full range of proteins that a cell is able to produce
Anticodon
- 3 bases on the tRNA which are complementary to the codon on mRNA
mRNA structure
- single-stranded
- made up of codons
- a copy of one gene
tRNA structure
- single-stranded, folded to create a cloverleaf shape
- held in place by hydrogen bonds
- has an anticodon and amino acid binding site
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mRNA function
- a copy of a gene from DNA
- created in the nucleus, and it then leaves the nucleus
- to carry the copy of the genetic code of one gene to a ribosome in the cytoplasm
tRNA function
- a specific amino attaches at the binding site
- transfers this amino acid to the ribosome to create the polypeptide chain
Transcription
- the first stage in protein synthesis
- one gene in DNA is copied into mRNA
- occurs in the nucleus
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Translation
- the second stage in protein synthesis
- the polypeptide chain is created using both the mRNA base sequence and the tRNA
- occurs on ribosomes in the cytoplasm
Which enzymes are involved in transcription?
- DNA helicase
- RNA polymerase
DNA helicas
- catalyses the breaking of hydrogen bonds between the two strands of DNA
RNA polymerase
-
joins adjacent RNA nucleotides
together - forming a phosphodiester bond
pre-mRNA
- mRNA in eukaryotes that still
contains the introns
How is pre-mRNA modified?
- the introns are removed by a protein called a spliceosome
- this leaves just the exons
What is ATP used for in translation?
- forming the peptide bond between amino acids
Haploid
- one copy of each chromosome in a cell
Diploid
- two copies of each chromosome in a cell
Meiosis
- cell division that creates
genetically different gametes - there are two nuclear divisions in this process
- results in four haploid daughter cells
Independent segregation
- homologous pairs of chromosomes randomly line up opposite each other at the equator of the cell
- when they separate it creates a large number of possible combinations of chromosomes in the daughter cells produced
Crossing over
- homologous pairs of chromosomes line up opposite each other at the equator in meiosis 1
- parts of the chromatids twist, break and recombine with another chromatid
- results in new combinations of alleles in the gametes
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Gametes
- sex cells (sperm and egg)
How does meiosis introduce variation?
- crossing over
- independent segregation
Types of gene mutations
- deletion
- substitution
What is a frameshift?
- the removal of one base
changes all of the subsequent codons - all the bases shift back one position
Chromosome mutation
- change in the number of chromosomes
- occurs during meiosis
Deletion mutation
- a gene mutation
- a base is removed from a sequence
- causes a frameshift
Substitution mutation
- a gene mutation
- a base is swapped for a different one
Non-disjunction
- the chromosomes or chromatids do not split equally during anaphase
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Polyploidy
- changes in whole sets of chromosomes
- e.g. when organisms have three or more sets of chromosomes
- mainly occurs in plants
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Aneuploidy
- a change in the number of individual chromosomes
- e.g. 3 copies of chromosome 21
Down’s syndrome
- caused by a chromosome mutation
- an example of aneuploidy
- e.g. 3 copies of chromosome 21
Genetic diversity
- the number of different alleles in a population
Natural selection
- the process that leads to evolution in populations
- results in species becoming better adapted to their environment
Gene pool
- all the genes and alleles in a population at a particular time
Allele frequency
- the proportion of organisms within the population carrying a particular allele
Evolution
- the change in allele frequency
- over many generations in a population
Selection pressure
- factors that affect the survival of an organism
- the driving force of natural selection
Types of selection
- stabilising
- directional
Directional selection
- one of the extreme traits has a selective advantage
- occurs when there is a change in the environment
- the modal trait changes
Stabilising selection
- the middle (median) trait has a selective advantage and continues to be the most frequent in the population
- represented by a normal distribution graph
- range decreases as the extreme traits are lost over time
An example of directional selection
- antibiotic resistance
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An example of stabilising selection
- human birth weight
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Definition of a species
- a group of similar organisms that can breed to make fertile offspring
Binomial system
- a universal naming system
- individuals are named after their genus and species
- e.g. Homo sapiens
Courtship behaviour
- different sequences of behaviours that animals demonstrate to attract a mate
- each species demonstrates its own unique behaviour
- essential for successful mating
Importance of courtship
- helps to synchronise mating behaviour so that the animals are more likely to mate when the female is releasing eggs
- increases the likelihood of successful mating
- enables individuals to recognise members of the same species and opposite sex
What is meant by a hierarchy?
- smaller groups arranged within larger groups
- there is also no overlap between groups
Phylogenetics
- the study of species’ evolutionary origins and relationships
Common ancestor
- the species from which another species evolved
Taxon
- the term for each group in classification
What are the taxa in the Linnaeus classification system?
- domain
- kingdom
- phylum
- class
- order
- family
- genus
- species
Species diversity
- the number of different species and the number of individuals within each species in a community
Species richness
- the number of different species in a particular area at the same time
Index of diversity
- a measure of species diversity
- a calculation to measure the relationship between the number of species in a community and the number of individuals in each species
Community
- all the species in a particular area at a particular time
Environment
- the abiotic (non-living) factors in an area
Habitat
- The range of physical, biological and environmental factors in which a species can live
Which farming techniques reduce biodiversity?
- destruction of hedgerows
- selective breeding
- monocultures
- over-grazing
- filling in ponds and draining wetlands
Biodiversity
- a measure of the range of habitats
- from a small local habitat to the entire Earth
- a measure of species diversity
- a measure of genetic diversity
How is standard deviation more useful than a mean?
- it gives you the spread of data (variance) around the mean\
- overlaps in standard deviation indicate no significant difference in the means
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How do you make sure your sampling is representative of the population?
- take a large sample (at least 30)
- randomly sample to avoid bias
How can you measure genetic diversity?
- by comparing the base sequence of DNA, mRNA and the amino acid sequence of the proteins
Genetic diversity
- the mesure of how many different alleles there are for each gene
Formula for Index of Diversity
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Compare the DNA in chloroplast/mitochondria to prokaryotic DNA
- the DNA is similar to prokaryotic DNA, in that it is:
- short
- circular
- not histone bound
How does transcription differ between prokaryotes and eukaryotes?
- Transcription in prokaryotic organisms results in mRNA that requires no modification
- because prokaryotic DNA does not contain any introns
- eukaryotic transcription results in pre-mRNA that needs introns removed
What is the role of ribosomes in translation?
- it holds two tRNA molecules
- to enable peptide bonds to form between the amino acids
What is the role of tRNA in translation?
- it carries a specific amino acid
- anticodons align opposite complementary codons on the mRNA
Gene mutations
- a change in the base sequence of DNA
- substitution or deletion
When in the cell cycle do gene mutations occur?
- interphase (S phase)
- this is when DNA is replicated
