Genetic information, variation and organism relationships

• Compare DNA in eukaryotic and prokaryotic cells

  SIMILARITIES

  • nucleotide structure is identical: deoxyribose attached to phosphate and a base, adjacent nucleotides joined by phosphodiester bonds and complementary bases joined by hydrogen bonds

  DIFFERENCES

  • Eukaryotic is longer
  • Eukaryotic is linear, Prokaryotic is circular
  • eukaryotic has his tones, prokaryotic doesnt
  • eukaryotic contains introns, prokaryotic doesn’t
• What’s a chromosome?

  Long linear DNA and its associated histone proteins in the nucleus of eukaryotic cells
• What is a gene?

  A sequence of DNA bases is that code for the amino acid sequence of a polypeptide or a functional RNA
• What is a locus?

  Fixed position a gene occupies on a particular DNA molecule
• What’s a triplet?

  A sequence of 3 DNA bases coding for a specific amino acid
• What is universal?

  the same base triplets code for the same amino acids in all organisms
• What is non-overlapping?

  Each base pair is part of only one triplet so each triplet it read as a discrete unit
• What is degenerate?

  An amino acid can be coded for by more than one base triplet
• What are non-coding base sequences and where are they found?

  DNA that does not code for amino acid sequences/polypeptides and you can find them between genes (non-coding multiple repeats) or within genes (introns)
• What’s an exon?

  Base sequence of a gene coding for amino acid sequences in a polypeptide
• What’s an intron?

  Base sequence of a gene that doesn’t code for amino acids in eukaryotic cells
• What’s a genome?

  The complete set of genes in a cell
• What’s a proteome?


  The full range of proteins that a cell can produce
• What are the two stages of protein synthesis?

  Transcription

  • Production of mRNA from DNA in the nucleus

  Translation

  • production of polypeptides from the sequence of codons carried by mRNA at ribosomes
• What’s the differences and the similarities of tRNA and mRNA?

  SIMILARITIES

  • both single polynucleotide strand

  DIFFERENCES

  • tRNA is folded into a clover shape whereas mRNA is linear
  • tRNA has hydrogen bonds between base pairs mRNA doesnt
  • tRNA is shorter fixed length but mRNA is longer variable length
  • tRNA has anticodons mRNA has codons
  • tRNA has an amino acid binding site mRNA doesn’t
• How is mRNA formed by transcription in eukaryotic cells?

  1. DNA helicase breaks hydrogen bonds between base pairs
  2. only one DNA strand acts as template
  3. free nucleotides align next to complementary base on template strand
  4. RNA polymerase joins adjacent nucleotides
  5. this forms phosphodiester bonds via condensation reaction
  6. pre-mRNA is formed and this is spliced to remove introns forming mRNA
• How is the production of mRNA in eukaryotic cell different to prokaryotic cell?

  Pre-mRNA is produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells Because genes in prokaryotic cells don’t contain introns so no splicing
• Describe how translation leads to the production of a polypeptide?

  1. mRNA attaches to a ribosome and moves to a start codon
  2. tRNA brings a specific amino acid
  3. tRNA anticodon binds to complementary mRNA codon
  4. Ribosome moves along to the next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond using energy from hydrolysis of ATP
  5. tRNA released after amino acid joined polypeptide
  6. ribosome moves along mRNA to form the polypeptide until a stop codon is reached

  
• What’s the role of ATP in translation?

  Hydrolysis of ATP to ADP + Pi releases energy so amino acids join to tRNAs and peptide bonds form
• What’s the role of tRNA in translation?

  Attached to a specific amino acid in relation to anticodon and complementary base pairs to mRNA codon forming hydrogen bonds. 2 tRNAs bring amino acids together so peptide bonds can form
• What’s the role of ribosomes in translation?

  mRNA binds to ribsosome with space for 2 codons and allows tRNA anticodon to bind. Also catalyses the formation of peptide bin between amino acids and moves along mRNA to next codon (translocation)
• What is a gene mutation?

  A change in the base sequence of DNA and can arise spontaneously during DNA replication
• What’s a mutagenic agent?

  A factor that increases rate of gene mutation
• How can gene mutation lead to non functional protein or enzymes?

  1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
  2. so changes sequence of amino acids in the polypeptide
  3. so changes position of hydrogen/ionic/disulphide bonds
  4. so changes proteins tertiary structure
  5. enzymes active site changes shape so substrate cant bind
• Explain effects of a substitution mutation?

  1. base/nucleotide in DNA replaced by a different base
  2. this changes one triplet so changes one mRNA codon
  3. so one amino acid in polypeptide changes so tertiary structure may change or amino acid doesn’t change

  due to degenerate nature of genetic code or if mutation is in an intron
• Explain the possible effects of a deletion mutation?

  1. One base removed from DNA sequence
  2. changes sequence of DNA triplets from point of mutation
  3. changes sequence of mRNA codons after point of mutation
  4. changes amino acid sequence in primary structure
  5. Changes position of bonds in tertiary structure
  6. changes tertiary structure/shape of protein
• What are the features of homologous chromosomes?

  Same length same genes at same loci but may have different alleles
• What’s the difference between diploid and haploid?

  Diploid has 2 complete sets of chromosomes represents as 2n

  Haploid has a single set of unpaired chromosomes represents as n
• How does a cell divide by meiosis?

  meiosis 1 separates homologous chromosomes

  • chromosomes arrange into homologous pairs
  • crossing over between homologous chromosomes
  • independent segregation of homologous chromosomes

  Meiosis 2 separates chromatids
• Why is the number of chromosomes halved during meiosis?

  Homologous chromosomes are separated during meiosis 1
• How does crossing over create genetic variation?

  • Homologous pairs of chromosomes form a bivalent
  • chiasmata form
  • alleles equal lengths of non-sister chromatids exchanged between chromosomes
  • creating new combinations of alleles on chromosomes
• How does independent segregation create genetic variation?

  • Homologous pairs randomly align at equator so random as to which goes into each daughter cell
  • creating different combinations of maternal and paternal chromosomes/alleles in daughter
• How is genetic variation within a species increased without mutation and meiosis?

  Random fertilisation/fusion of gametes

  Creating new allele combinations/ new maternal and paternal chromosome combinations
• Explain the difference between mitosis and meiosis?

  Mitosis produces 2 daughter cells whereas meiosis produces 4 (2 divisions)

  mitosis maintains the chromosome number whereas meiosis halves the chromosome number as homologou pairs separate

  mitosis produces genetically identical whereas meiosis produces genetically different as crossing over and segregation happen
• What’s the importance of meiosis?

  Two divisions creates haploid gametes so diploid number is restored at fertilisation and chromosome number maintained between generations.

  Independent segregation and crossing over creates genetic variation
• How can you recognise where meiosis and mitosis occur in a life cycle?

  Mitosis occurs between stages where chromosomes number is maintained whereas meiosis occurs between stages where chromosome number halves
• How can mutations in the number of chromosomes arise?

  • Spontaneously by chromosome non-disjunction during meiosis
  • homologous chromosomes or sister chromatids fail to separate during meiosis so some gametes have an extra copy of a particular chromosome and others have none
• Non disjunction

  
• How can the number of possible combinations of chromosomes in daughter cells followinb meiosis can be calculated?

  2^n where n = number of pairs of homologous chromosome (half diploid number)
• How can the number of possible combinations of chromosomes following random fertilisation of two gametes can be calculated?

  (2^n)^2 where n = number of pairs of homologous chromosomes (half diploid number)