Genetics and Variation

• Two main roles of DNA


  -Replication so cell can divide of growth and repair-Codes for proteins (protein synthesis)
• What is gene?

  Section of DNA with a specific base sequence which codes for a particular polynucleotide (location=locus)
• Three types of RNA

  mRNA= copies the gene (transcription)rRNA = helps code for ribosomestRNA= brings the amino acid

  
• Features of the genetic code

  -Nonoverlapping-Read in triplets--Universal-Degenerate
• Three bases in DNA, mRNA and tRNA

  
• Protein Synthesis (Stage 1: Transcription)


  - DNA helicase unwinds and separates the two Polypeptide chains by breaking hydrogen bonds between bases- one of these strands is used as a template Strand-Free ,activated RNA nucleotides in the nucleoplasm are attracted to and hydrogen bond to their complementary base pairs- RNA polymerase forms phosphodiester bonds between adjacent nucleotides to form the sugar phosphate backbone- This continues until they reach a stop triplet ( sequence of 3 bases )- This forms mRNA

  
• Slicing of pre-mRNA into mature mRNA

  
• Transfer RNA

  
• Translation


  1. mRNA associates with a ribosome 2. Ribosome moves to the start codon- AUG3. tRNA brings specific amino acid4. Anticodon to codon interactions5. Ribosome moves along to next codon6. Process repeated and amino acids join by peptide bonds to form polypeptide7. Uses energy from ATP

  
• Degenerate codon table

  
• What is a genome?


  the complete set of genes or genetic material present in a cell or organism
• What is a proteome?


  The complete set of proteins that can be produced by a cell
• Differences in transcription between prokaryotes and eukaryotes

  1. Splicing only occurs in eukaryotes 2. Introns only removed in prokaryotes
• What is a mutation?

  An alteration to the DNA base sequence. Often arise spontaneously during DNA replication.
• Common types of mutations

  Gene MutationsPoint mutations (base substitutions)Base deletions (frame shift)Chromosome Mutations
• Why might a mutation not lead to change in amino acid sequence?

  ● Genetic code is degenerate so mutation may end up coding for same amino acid as the original triplet. ● Mutation may occur in intron.
• What a deletion mutation?

  -When a nucleotide in the base sequence is lost-Causes a frame shift-More likely to be significant -Complete change in amino acid sequence
• What is a polyploidy chromosome mutation?


  Where an individual has three or more sets of chromosomes instead of two.
• What is a chromosome non-disjunction?

  -When chromosomes fail to separate correctly in meiosis-Resulting in gametes with one more or less chromosome than normal
• Stages of meiosis

  Meiosis I = Homologous pairs divideProphase I, Metaphase I, Anaphase I, Telophase I,Meiosis II = Chromatids divideProphase II, Metaphase II, Anaphase II, Telophase II

  
• What does meiosis produce?

  4 genetically different haploid cells
• What is the importance of meiosis?

  -Ensures that when gametes fuse during fertilisation, normal diploid number of chromosomes are produced
• What are homologous chromosomes?


  Pair of chromosomes with the same gene loci
• What is a Locus?


  Location of a gene on a chromosome
• What is a zygote?

  fertilized egg cell
• What is an allele?

  Different type of the same gene
• Meaning of n in meiosis

  
• Two ways which meiosis causes genetic variation

  -Independent segregation -Crossing over
• What is independent segregation?


  Refers to the random division of homologous pairs, and their position along the equator

  
• Formula to work out how many possibilities due to just independent segregation and independent segregation and random fertilisation


  2 to the power of how many pairs you have(2^n)^2
• How does crossing over contribute to genetic diversity?

  -Homologous pairs of chromosomes associate-Chiasmata forms-Exchange of alleles between chromatids-Therefore cells end up with different combinations of alleles

  
• Define genetic diversity


  The total number of different alleles in a population
• Advantage of high genetic diversity


  Ability to adapt to a change in environment; allows natural selection to occur.
• What is synapsis?


  Pairing of homologous chromosomes
• What is bivalent?

  A pair of homologous chromosomes
• What is the Chiasmata?


  The point where chromosomes crossover
• Other sources of variation

  -Mating is random-Potential mutations
• Stages of Natural Selection

  Stage 1There is variation present within a population due to random mutations producing new alleles in a geneStage 2Selection pressures arise due to competition for space, food, water, mates etcStage 3Some mutations may give rise to advantageous alleles, so are more likely to survive and reproduce, so causes an increase in the frequency of the advantageous alleles in the population, so are more adapted to the environment
• Continuous and discontinuous variation

  
• Types of selection