Genes, Alleles, and Chromosomes

• Genetic information, variation, and relationships between organisms are explored

Gene

• A gene is a DNA section that codes for a polypeptide and functional RNA
• The code is a specific sequence of bases
• Polypeptides create proteins; thus, genes determine an organism's proteins
• A gene's location on a chromosome is the locus

Allele

• Is an alternative form of a gene
• Most genes occur in pairs, but some have multiple forms

Chromosome

• In eukaryotic cells, DNA is stored chromosomes
• Humans have 23 pairs, totaling 46 chromosomes

Homologous Chromosomes

• Matching chromosome pairs are homologous
• Homologous pairs are the same size and have the same genes, but alleles may differ

DNA Storage in Eukaryotic Cells

• DNA is stored as chromosomes in the nucleus
• Chromosomes are linear, tightly coiled, and wrapped around histones

DNA Storage in Prokaryotes

• Prokaryotes have shorter, circular DNA
• The DNA is not wound around histones and instead is supercoiled

DNA in Mitochondria and Chloroplasts

• Mitochondria and chloroplasts contain additional DNA in eukaryotic cells
• This DNA is similar to prokaryotic DNA because it is short and circular

Start and Stop Codons

• The start codon has the three bases that initiate translation
• The stop codon has three bases that don't code for an amino acid, marking the end of a polypeptide chain and stopping translation

The Genetic Code

• An amino acid is coded for by a three-DNA base known as the "triplet code"
• On mRNA, a sequence of three bases coding for an amino acid is a codon
• The genetic code is degenerate, universal, and non-overlapping

Degenerate

• There are 64 possible triplet combinations for 20 amino acids
• Each amino acid is coded for by more than one triplet of bases

Universal

• The same triplet of bases code for the same amino acid in all organisms
• This means genetic engineering is possible

Non-Overlapping

• Each base in a gene is part of one triplet of bases that codes for one amino acid
• This means each codon is read as a discrete unit
• A point mutation will only affect one codon and one amino acid

Introns and Exons

• Introns are DNA sections that don't code for amino acids in polypeptide chains and are in eukaryotic, but not prokaryotic DNA
• Introns get removed (spliced) from mRNA molecules
• Exons are sections of DNA that code for amino acids

Genome and Proteome

• The genome is a cell's complete set of genes
• A proteome is the full range of proteins a cell can produce
• Bacteria contains 600,000 DNA base pairs
• Humans contain 3 billion base pairs

Messenger RNA (mRNA)

• mRNA is a gene copy from DNA
• Made in the nucleus, it carries the genetic code of one gene to a ribosome in the cytoplasm
• mRNA is shorter, only the length of one gene
• It is single-stranded
• Every three bases in the sequence code for a specific amino acid, called codons

Transfer RNA (tRNA)

• It is only in the cytoplasm
• tRNA is single-stranded and folded into a cloverleaf shape
• Held in place by hydrogen bonds, tRNA attaches to one of the 20 amino acids and transfers it to the ribosome

Protein Synthesis

• It happens on ribosomes and production from the DNA code occurs in two stages: transcription and translation

Transcription

• A gene on the DNA is copied into mRNA
• It occurs in the nucleus
• Helicase unwinds the DNA to expose bases as a template
• Only one DNA chain acts as a template
• Helicase breaks the hydrogen bonds between bases
• Free mRNA nucleotides align opposite the exposed complementary DNA bases
• RNA polymerase bonds the RNA nucleotides together to create a new RNA polymer chain
• One entire gene is copied

Prokaryotes

• Transcription produces a finished mRNA molecule without modification
• Prokaryotic DNA doesn't contain introns

Pre-mRNA in Eukaryotes

• Eukaryotic DNA has introns that are to be removed before translation
• Following transcription, the mRNA is pre-mRNA and contains introns
• This gets modified with a spliceosome to become mRNA that leaves the nucleus

Translation

• The polypeptide chain is created from the mRNA sequence and the tRNA
• mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm at the start codon
• tRNA with the complementary anticodon aligns opposite the mRNA
• The ribosome goes along the mRNA and enables each tRNA to attach
• Two amino acids are joined by a peptide bond catalyzed by an enzyme using ATP
• It happens until the ribosome reaches the stop codon
• The ribosome detaches and translation ends
• The polypeptide chain enters the Golgi body for folding and modification

Genetic Variation

• It can be introduced in meiosis, mutations, and the random fertilisation of gametes
• Meiosis creates genetically different gametes with two nuclear divisions, resulting in four haploid daughter cells
• Meiosis involves independent segregation and crossing over

Independent Segregation

• The homologous pairs of chromosomes line up opposite at the equator in meiosis I
• Which side of the equator the maternal chromosomes lie on is random
• Pairs separate, and one of each homologous pair ends up in the daughter cell
• Results in a large number of possible chromosome combinations

Crossing Over

• Occurs in meiosis I when homologous pairs line up opposite each other at the equator
• Chromatids become twisted and put tension on each other so pairs of chromatids break
• Broken chromatid parts recombine with another, resulting in new allele combinations

Gene Mutation

• A gene mutation is a change in the DNA base sequence, randomly occurring during DNA replication
• Random mutations occur from mutagenic agents, such as high-energy radiation, ionising radiation, and chemicals
• A base can be deleted or substituted
• A base substitution can be 'silent', coding for the same amino acid
• Base deletions result in a frameshift, changing subsequent codons, making mutations more harmful

Chromosome Mutations

• Mutations in the number of chromosomes arise spontaneously by nondisjunction during meiosis

Non-Disjunction

• Occurs when chromosomes or chromatids don't split equally during anaphase

Aneuploidy

• A change in the number of individual chromosomes
• Individual homologous chromosome pairs fail to separate during meiosis
• It results in a gamete with one more or fewer chromosomes

Polyploidy

• A change in whole sets of chromosomes when organisms have three or more sets

Genetic Diversity

• Genetic diversity increases via meiosis through independent segregation and crossing over
• Genetic diversity increases through gene and chromosome mutations

Natural Selection

• It's the process leading to evolution with the change in allele frequency during generations
• It results with a better adaptation to the environment

Adaptions

• Can be anatomical, physiological, or behavioural
• Anatomical adaptions are spiked on a hedgehog
• Physiological adaptions are hedgehogs hibernating and slowing metabolic reactions in winter
• Behavioural adaptions are curling up when in danger

Natural Selection: 5-Mark Question

1. Random mutations occur
2. It introduces genetic variation
3. Some mutations give an advantage to the organism for survival
4. The new allele provides a reproductive selective advantage to pass on to their offspring
5. Frequency of this allele will increase

Directional Selection: Extreme Trait Advantages

• Directional selection is when one of the extreme traits has a selective advantage
• It occurs from a change in the environment, changing the modal trait

Stabilising Selection: Selective Advantage

• Stabilising selection is when the middle (median) trait has a selective advantage
• It becomes the most frequent in the population and represented by a normal distribution graph
• It occurs when there is no change in the environment, remains the same
• The extreme traits decline

Courtship

• Behaviour includes the different behaviours displayed to attract a mate, they are unique and essential for successful mating
• They synchronize mating behaviour in individuals until they are sexually mature for fertile eggs from the female
• This increases the likelihood of selecting a strong, healthy mate
• Includes dance moves, sounds, pheromones, or displays of feathers to pass on successful alleles

Binomial System

• Is universal for species using the genus and species
• Helps different names not be used in different countries to reduce errors

Phylogenetic Classification

• Arranges species into groups using their evolutionary origins and relationships
• Shows who's related to whom with common ancestors on a phylogenetic tree
• The DNA base sequence and the amino acids in the polypeptide chain of proteins help measure this relationship
• They are called physical and biochemical similarities to aid a heirarchy

Classification Heirarchy

• One example classification: Domain, kingdom, phylum, class, order, family, genus, species
• Each level is known as a taxon (taxa plural) and aid in understanding relationships, helping with changes that may occur

Biodiversity

• The different meanings: species diversity, genetic diversity, and ecosystem diversity

Species Richness

• Number of different species in a community that occur at certain times

Communities

• Is where all the organisms are living at a specific time

Human Impact

• It's caused by human activity of farming which reduces biodiversity from Destruction of hedgerows, breeding, monocultures, over-grazing, and filling in ponds as well as draining wetlands
• This reduces habitats and food sources as conservation measures put in place

How to Measure Biodiversity

• The index of diversity is a measure of species diversity using species and the number of individuals in each species

Genetic Diversity

• The measure of alleles for each gene measured by observable the DNA, mRNA and the amino acid sequence
• Samples are taken for this population and ensure this sample is representative To avoid bias you should randomly sample