3.4 Genetic Information, Variation and Relationships between Organisms Flashcards
Define and outline a gene
• A section or base sequence of DNA located at a specific locus on a DNA molecule
• The base sequence of a gene carries coded genetic information that determines the amino acid sequence of a polypeptide and functional RNA
Identify what causes adaptation
Genetic diversity being acted upon by natural selection
How can variation be measured within a species
Using differences in the base sequence of DNA or in the amino acid sequence of proteins
How can biodiversity be measured
• Species richness
• An index of diversity
Outline Prokaryotic DNA
DNA molecules are short, circular and not associated with proteins
Outline Eukaryotic DNA
• In the nucleus, DNA molecules are very long, linear and associated with proteins called histones
• Together a DNA molecule and it’s associated proteins form a chromosome
Outline DNA in the mitochondria and chloroplasts of Eukaryotic cells
Like prokaryotic DNA, is short, circular and not associated with proteins
Identify the differences between Prokaryotic and Eukaryotic DNA
• Eukaryotic DNA’s in the nucleus while prokaryotic DNA’s free in the cytoplasm
• Eukaryotic DNA’s linear while prokaryotic DNA’s circular
• Eukaryotic DNA’s longer the prokaryotic DNA
• Eukaryotes have exons and introns, prokaryotes only have exons
• Eukaryotic DNA’s wrapped around/associated with proteins called histones, prokaryotic DNA isn’t associated with proteins
Define an exon
Sections of DNA that are always expressed in a protein
Define an intron
Sections of DNA that don’t code for anything
Define and outline an allele
• One of alternate forms of the same gene
• Each individual inherits one allele from each parent that are dominant or recessive and may be the same or different
• If different, there’s a difference in the base sequence so there’s a different amino acid sequence producing a different polypeptide
Define a locus
The fixed position of a gene on a particular DNA molecule
How many amino acids are there that regularly occur in proteins
20
What are the four DNA bases
• Adenine
• Thymine
• Cytosine
• Guanine
How are amino acids coded for
A sequence of three DNA bases called a triplet or codon that codes for a specific amino acid
Identify the main features of the genetic code
• Degenerate; some amino acids are coded for by more than one codon
• Universal; the same codons code for the same amino acids in every organism (indirect evidence for evolution)
• Non-overlapping; each base is only read once in a codon
• Triplet; 3 DNA bases, triplet of bases code for one amino acid (codon)
What’s a codon
A triplet of bases coding for an amino acid
Identify three features of codons within the genetic code
• The start of the sequence is always methionine
• Three codons don’t code for any amino acids and act as stop codons signalling the end of a polypeptide chain
• Much Eukaryotic nuclear DNA doesn’t code for proteins (introns)
Define and outline chromosomes
• Sister chromatids held together by the centromere
• Visible during cell division, but for the rest of the time are widely dispersed throughout the nucleus
Outline haploid cells
• Cells with one set of chromosomes
• The result of meiosis
• All gametes (sex cells) are haploid
Outline diploid cells
• Cells with homologous chromosomes
• Created through mitosis
• All somatic (body cells) are diploid
Define the genome
The complete set of genes it genetic material present in a cell or organism
Define the proteome
The full range of proteins that a cell is able to produce
Outline the structure and function of messenger RNA (mRNA)
• Structure:
- Single strand, single gene long (size depends on
the length of the gene) with bases in triplets
• Function:
- Made during transcription, carries the genetic
code to the ribosome that is translated into a
polypeptide
Outline the structure and function of transfer RNA (tRNA)
• Structure:
- Single strand, folded into a clover leaf with
hydrogen bonds between complimentary bases
- Always the same size
- Three free bases, with the anticodon at one end
and the amino acid bonding site at the other
• Function:
- Carries amino acids to the ribosome during
translation
Identify the two processes is protein synthesis split into in order
• Transcription
• Translation
Define transcription, referencing the difference between prokaryotes and eukaryotes
The production of mRNA from DNA
• In prokaryotes, transcription results directly in the production of mRNA from DNA
• In Eukaryotes, transcription results in the production of pre-mRNA which is then spliced to form mRNA
Describe the process of transcription in prokaryotes
• Hydrogen bonds between complimentary DNA strands are broken
• One strand acts as the template strand
• RNA nucleotides will align by complimentary base pairing, (uracil-adenine/cytosine-guanine)
• RNA Polymerase joins adjacent nucleotides together
• The DNA helix joins back together to form mRNA
Describe the process of transcription in eukaryotes
• Hydrogen bonds between complimentary DNA strands are broken
• One strand acts as the template strand
• RNA nucleotides will align by complimentary base pairing, (uracil-adenine/cytosine-guanine)
• RNA Polymerase joins adjacent nucleotides together
• The DNA helix joins back together and pre-mRNA’s modified by splicing where pre-mRNA is placed to form mRNA
Describe the process of splicing in eukaryotic transcription
• Introns are removed and exons are spliced back together in order
• pre-mRNA is spliced to form mRNA
Define translation
The production of polypeptides from the sequence of codons carried by mRNA
Describe the process of translation
• mRNA associates with a ribosome
• tRNA with a complimentary anticodon, binds to the first mRNA codon
• The tRNA brings a specific amino acid, and the second tRNA brings a second specific amino acid and so on
• Using ATP, the ribosome catalyses a peptide bond between amino acids
• The first tRNA’s released from the ribosome, which moves along the mRNA to the next codon and the process repeats until it reaches a stop codon
Outline the structure and function of a ribosome during translation
• Structure: Formed of protein and RNA and made up of a large and small subunit, that fit either side of the mRNA strand, they can be free in the cytoplasm or attached to the endoplasmic reticulum
• Function: Site of protein synthesis via translation, the large subunit joins amino acids and the small subunit contains the mRNA binding site
How many ribosomes can be found in a mammalian cell
Up to 10million
Outline the role of ATP in translation
• Small and soluble, so it can be transported around the cell
• ATP can’t pass out of the cell so cells always have an immediate supply of energy
• Hydrolysis is a single reaction, so energy’s released quickly and it rapidly resynthesises
• Low activation energy so it can release stored energy quickly
• Stores and releases small amounts of energy at a time, so no energy’s wasted
• Can transfer energy from one molecule to another by transferring one of its phosphate groups
Why can’t DNA leave the nucleus
It’s too large and associated with proteins called histones
Outline meiosis
• Meiosis is a form of reduction division that produces daughter cells that are genetically different from each other
• Two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell
• Genetically different daughter cells results from the independent segregation of homologous chromosomes
• Crossing over between homologous chromosomes results in further genetic variation among daughter cells
• Independent segregation of chromosomes and crossing over of homologous chromosomes increase variation in meiosis because of the new combinations of alleles in gametes.
• New alleles are only caused by mutations that can only be passed on in meiosis, not mitosis
Identify gametes in different eukaryotes
• Sperm and ova in animals
• Pollen and ova in plants
Define reduction division
The chromosome number’s reduced and haploid cells are formed
Describe the process of meiosis
Prophase I
• Replicated chromosomes condense to form
chromatin and become visible
• Crossing over can occur between homologous
chromosomes
Metaphase I
• Homologous chromosomes line up randomly in
pairs on the equator of the cell through
independent segregation
Anaphase I
• Spindle fibres, made by centrioles, attach to the
centromeres and chromosomes are pulled to
opposite ends of the cell. Homologous
chromosomes are now separated
Telophase I
• Chromosomes have gathered at either end of
the cell and cytokinesis occurs
MEIOSIS II
Prophase II
• Two separate cells
Metaphase II
• Chromosomes mine up on the equator of the
cell, one on top of eachother
Anaphase II
• Spindle fibres attach to the centromeres and pull
in the opposite directions. Centromeres break,
sister chromatids are separated and pulled to
opposite poles of the cell
Telophase II
• Cytokinesis occurs and four non-identical
haploid gametes (sex cells) are produced
Describe the process of crossing over during Prophase I of meiosis
• Crossing over can occur between homologous chromosomes, but doesn’t always happen
• Homologous chromosomes ‘cross over’ each
other to form a chiasma. This produces new
combinations of alleles, increasing variation in
meiosis
Define homologous chromosomes
• A pair of chromosomes with the same genes at the same loci, but are not identical.
• For example, one of the homologous chromosomes may have the gene for eye colour at ‘point A’ for blue eyes while the other homologous chromosome will also have the gene for eye colour at ‘point A’ but will have an allele for brown eyes
Outline cytokinesis
• The separation of the parent cell into two genetically identical daughter cells once a new nucleus has completely reformed at each like if the parent cell after telophase
• In animals, a ‘cleavage furrow’ forms and separates the daughter cells but in plants a ‘cell plate’ forms at the metaphase plate. Once it reaches the walls of the parent cell, new cell walls are produced, separating the new daughter cells
Define and outline mutations
• A random change in the base sequence of chromosomes. They can arise spontaneously during DNA replication (mitosis/meiosis)
• Mutations often have no effect but can be harmful or beneficial. Types of mutation can include base deletion and base substitution mutations. Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
• Mutagenic agents can increase the rate of gene mutation
Outline base substitution mutations
• When one nucleotide is replaced by a different nucleotide E.g. TAG becomes TCG
• There are three possible consequences ;
- Nonsense mutations: The codon could be changed to a stop codon
- Missense mutations: The codon could code for a different amino acid
- Silent mutation: The codon could code for the same amino acid. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids
Outline base deletion mutations
When one nucleotide is removed from a codon E.g. TAG becomes T G.
• This causes a frame shift of subsequent codons, because genetic code is triplet and non overlapping all following codons will be different
Outline chromosome mutations
• There are two categories caused by errors during meiosis:
• Polyploidy is when an organism has 3 or more sets of chromosomes instead of the usual 2 (diploid). This usually happens in plants and is catastrophically damaging in humans
• Chromosome non-disjunction is when chromosomes fail to separate during meiosis, gametes may end up with either no copy or an extra copy of a chromosome
Outline and describe the process of chromosome non-disjunction
• Chromosome non-disjunction is when chromosomes fail to separate during meiosis, gametes may end up with either no copy or an extra copy of a chromosome
• Homologous chromosomes don’t separate in meiosis I or sister chromatids don’t separate in meiosis II leading to gametes with an additional chromosome or one fewer chromosome (aneuploidy) this can cause:
• Live births with triosmy 13/18/21, but only 21 (Down syndrome) is survivable to adulthood
• Anaeuploidy
Outline Aneuploidy
• There are multiple types, Autosomal refers to body chromosomes (not X/Y sex chromosomes)
• Autosomal Aneuploidy is almost always fatal or causes a miscarriage
• Sex Chromosome Aneuploidy is survivable to adulthood (Common types include: XXY, XYY, XXX, XO)
• Mosaic Aneuploidy is where some cells have an additional chromosome
• Non-mosaic Aneuploidy is where all cells have an additional chromosome
Define genetic diversity
• The number of different alleles of genes in a population
• Genetic diversity is a factor enabling natural selection to occur
Identify the types of selection
• Directional selection
• Stabilising selection
• Disruptive selection
Describe directional selection and provide and example
• Selection for an extreme of a phenotype from the mean; anything that increases or decreases is directional selection
• E.g. selection for shorter fur in a warmer climate/ antibiotic resistance in bacteria
Describe stabilising selection and provide and example
• Selection for the mean phenotype; the frequency of the mean phenotype increases over time
• E.g. average body weight of babies when they’re born (too light/heavy leads to complications)
Describe disruptive selection and provide and example
• Selection for both extremes of a phenotype at the same time. This requires isolation between the population and leads to speciation.
• E.g. birds on an island with different beak sizes (thin beaks can eat insects, medium can eat fruit and thick can eat nuts/seeds). If there’s a drought, there’s no fresh fruit so only thick and thin beaks can survive. Eventually, they’ll be so genetically different they can’t interbreed, therefore there’s a new species
Outline natural selection and its principles
• Random mutation can result in new alleles of a gene
• Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success
• The advantageous allele is inherited by members of the next generation
• As a result, over many generations, the new allele increases in frequency in the population (increased allele frequency)
• Results in species that are better adapted to their environment. These adaptations may be anatomical, physiological or behavioural
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Outline classification of species
• A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships.
• It uses a hierarchy in which smaller groups are placed in larger groups, with no overlap between groups. Each group is called a taxon (pl. taxa)
• One hierarchy comprises the taxa: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
• Each species is universally identified by a binomial nam(e/ing system) made up by the genus and species e.g. Homo sapiens
• The binomial name’s always written in italics (underlined in handwriting). The genus always starts with a capital letter and the species with a lower case letter
Define a hierarchy
Smaller groups are placed in larger groups, with no overlap between groups
