Genetics, Biodiversity and classification

• Gene

  • A section of DNA that contains a code for making a polypeptide and functional RNA the code is a specific sequence of bases
  • Polypeptides make proteins and so genes determine the proteins of an organisms
  • The location of a particular gene on a chromosome is called the locus
• Allele

  • An allele is one of a number of alternative forms of a gene most genes occur in 2 occasionally more, different formed
• Chromosomes

  • In a eukaryotic cell nucleus, DNA is stored as chromosomes
  • Humans have 23 pairs of chromosomes, 46 in total
  • Pairs or matching chromosomes are called homologous pairs
  • A homologous air of chromosomes are exactly the same size have exactly the same genes but might have different alleles
• DNA storage in a eukaryotic

  • DNA is stored as chromosomes inside of the nucleus
  • Chromosomes in eukaryotic cells are linear in shape
  • To tightly coil the DNA to fit in the nucleus as chromosomes, the DNA is tightly wound around proteins called histones
  • This complex on DNA wrapped around a histone is called a nucleosome
• Start codon

  • At the start of every gene there is a 'Start codon' TAC in DNA or AUG in mRNA this codes for the amino acid methionine this methionine is later removed from the protein if it is not actually needed for the structure
• Stop codon

  • At the end of every gene there are 3 bases that do not code for an amino acid and is known as a 'stop codon' These stop codons mark the end of a polypeptide chain and causes ribosomes to detach and therefore stop translation They are ATT, ATC and ACT on DNA
• Degenerate

  • There are 20 amino acids that the genetic code has to be able to code for. There are 4 DNA bases and therefore 3 bases are needed to make enough combinations to code for at least 20 amino acids this can be proven mathematically (4^n)
• Universal

  • The same triplet of bases codes for the same amino acid in all organisms this is why the genetic code is described as being universal
  • Advantage as it means genetic engineering is positive
• Non-overlapping

  • Each base in a gene is only part of one triplet of bases that codes for one amino acid therefore each codon or triplet of bases is read as a discrete unit
  • Advantage as is a point mutation occurs it will only affect one codon and therefore one amino acid
• Genome and proteome

  • The genome is an organism complete set of DNA in one cell, where as the proteome is the full range of proteins in one cell
  • The genome should never change whereas the proteome of the cell is constantly changing depending on which proteins are currently needed
  • The genome of organisms widely variers for example bacteria contain on average 6,000,000 DNA base pairs where as humans contain less base pairs
• Introns and Exons

  • Introns are sections of DNA that do not code for amino acids and therefore polypeptide chains. Introns are found in eukaryotes DNA, but not in prokaryotic DNA. These get removed, spliced out of mRNA molecules
  • Exons are the sections of DNA that do code for amino acids
• Proteins are created on ribosomes the production of proteins from the DNA code occurs in two main stages:

  1. Transcription = Where one gene on the DNA is copied into mRNA
  2. Translation = Where the mRNA joins with a ribosome and corresponding tRNA molecules brings the specific amino acid the codon codes for
• This is the stage in which the polypeptide chain is created using both the mRNA base sequence and the tRNA

  1. mRNA attaches to a ribosome in the cytoplasm
  2. Ribosomes attaches at the start codon
  3. The tRNA molecule with the complementary anticodon to the start codon
  4. The ribosomes will move along the mRNA molecule
  5. tRNA molecules are joined by a peptide bond this is catalysed by an enzyme and requires ATP
  6. Continues until the stop codon - Then the ribosome detaches
• The polypeptide chain is now created and will enter the Golgi body for folding and modification
• Transcription

  • A complementary mRNA copy of one genes of the DNA is created in the nucleus
  • mRNA is much shorted than DNA so it is able to carry the genetic code to the ribosomes in the cytoplasm to enable the protein to be made
  • DNA helix unwinds
  • One chain of the DNA acts as a template
  • catalysed by DNA helicase
  • Breaks the Hydrogen bonds
  • Free mRNA nucleotides in the nucleus align
  • RNA polymerase bonds together
• Once copied the mRNA is modified and then leaves the nucleus through the nuclear envelope pores
• Pre - mRNA and mRNA

  • Following transcription, Pre mRNA has to be modified to become mRNA that is ready to leave the nucleus and take part in translation
  • The introns are spliced out by a protein called a spliceosome this leaves behind just the exons, the coding regions
• • Meiosis produces daughter cells that are genetically different from each other
  • Meiosis involves two nucleus divisions and creates 4 haploid daughter cells from a single diploid parent cell.
• There are two mechanisms in meiosis which introduce variation

  • Crossing over between homologous chromosomes
  • Independent segregation of homologous chromosomes
  • Both occur in meiosis 1
• Independent segregation

  • In meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell
  • It is random which side of the equator the paternal and maternal chromosomes from each homologous pair
  • These pairs are separated so one of each homologous pairs ends up in the daughter cell
  • This creates a large number of possible combinations of chromosomes in the daughter cells produced
• Crossing over

  • When homologous pairs line up opposite each other at the equator in meiosis 1 parts of the chromatids can become twisted around each other
  • This puts tension on the chromatids causing pairs of the chromatids break
  • The broken parts of the chromatids recombine with the other chromatid
  • This result is new combination of alleles
• Meiosis compared to mitosis

  meiosis

  • Nuclear divisions
  • Haploid cell
  • Introduces genetic variation

  Mitosis

  • One nuclear division
  • Diploid cells
  • Creates genetically identical cells
• Final increase in genetic variation

  • Random fertilisation further increases genetic variation
  • There are 2n possible chromosomes combinations in gametes
  • It is random which egg and sperm will fuse in fertilisation therefore variation is increased
  • There is actually(2n)2 possible combinations of chromosomes when you consider random fertilisation
  • meiosis results in genetic variation
• • Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
  • Non-disjunction is when chromosomes or chromatids do not split equally during anaphase
• Chromosome non-disjunction

  • Changes in the number or structure of whole chromosomes can arise spontaneously due to chromosomes non-disjunction during meiosis
• Chromosome non-disjunction can occur in two forms:

  1. Changes in whole sets of chromosomes
  2. Changes in the number of individual chromosomes
• Polyploidy

  • Changes in whole sets of chromosomes occur when organisms have 3 or more sets of chromosomes rather then the usual two This condition is called polyploidy and mainly occurs in plants
• Aneuploidy

  • Changes in the number of individual chromosomes
  • Sometimes individual homologous pairs of chromosomes fail to separate during meiosis
  • This is non-disjunction and usually results in a gamete having one more or one fewer chromosome
  • On fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosome than normal in all their body cells
• Genetic diversity

  • It is the number of different alleles of genes in a population
  • Natural selection can only occur if there is genetic diversity within a population
• Natural selection

  • Natural selections is the process that leads to evolution in populations
  • Evolution is the change in allele frequently over many generations in a population
  • Natural selection results in species becoming better adapted to their environment
  • Adaptations may be anatomical physiological or behavioural
• Directional selection

  • One of the extremes has the selective advantage
  • Occurs when there is a change in the environment
  • The model trait changes
• Stabilising selection

  • The model traits has the selective advantage
  • Occurs when there is no change in the environment
  • The model trait remains the same
  • Standard deviation decreases, as individuals with the extreme trait decrease
• Courtship behaviour is essential for successful mating and for species recognition
• What is a species

  • A group of similar organisms that can breed to make fertile offspring
  • Species must reproduce and pass on advantageous alleles for the survival of their species
• Courtship rituals

  • A sequence of actions which is unique to each species this is how animals identify members of their own species to reproduce with
  • Most courtship rituals are performed by males
  • They can include a sequence of dana moves, sounds, release of pheromones, display of colour feathers or fighting
  • Females observe the courtship ritual and decide if they wat to mate with the male
• The importance of courtship

  • To ensure successful reproduction
  • enables them to recognise own species and opposite sex
  • Synchronises mating behaviour indicates sexually mature and in season
• Binomial system

  • First name is genus, second name is species
  • Same genus shows close relationships
• Why do different species look similar

  1. Live in a similar environment
  2. Have similar selection pressures
  3. Similar alleles will have the selective advantage
  4. Produces similar/same proteins and therefore have similar characteristics
• Classification systems

  • Classification systems are an example of a hierarchy
  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species
• Ways to classify

  • Originally based on visible similarities
  • More modern and accurate classification methods are:
  • DNA sequence
  • mRNA sequences
  • Amino acids sequence
  • Immunological