Genes, DNA and Chromosomes

• Describe how a gene is a code for production of a polypeptide?

  1. Base/nucleotide sequence
  2. In triplets
  3. Determines order of amino acid sequence
• Define the term exon?

  1. Base sequence coding for polypeptide
• Name the protein associated with DNA in a chromosome?

  1. Histone
• 3 differences between DNA in a plant cell and DNA in a prokaryotic cell?

  1. DNA in plants is associated with histones
  2. DNA in plants is longer than DNA in prokaryotes
  3. DNA in plants is linear, DNA in prokaryotes is circular
• Define 'non-coding base sequences' and describe where the non-coding repeats are positioned in the genome?

  1. DNA that does not code for protein
  2. Positioned between genes
• Give 3 ways in which the DNA in a chloroplast is different to the DNA in a nucleus?

  In chloroplasts

  1. DNA is shorter
  2. DNA circular not linear
  3. Fewer genes
• Why are some mutations silent?

  1. Triplets code for the same amino acid
  2. Occurs in introns
• Compare and contrast the DNA in eukaryotic cells and the DNA in prokaryotic cells?

  1. Nucleotide structure is identical
  2. Nucleotides joined by phosphodiester bond
  3. Eukaryotic DNA is longer
  4. Eukaryotic DNA is linear, prokaryotic DNA is circular
  5. Eukaryotic DNA contains introns, prokaryotic DNA does not
• What is a homologous pair of chromosomes?

  1. Two chromosomes that carry the same genes
• Describe 2 differences between the structure of a tRNA and mRNA molecule?

  1. tRNA is clover leaf shaped, mRNA is linear
  2. tRNA has hydrogen bonds, mRNA does not
• Describe and explain a difference between the structure of mRNA and pre-mRNA?

  1. mRNA has no introns
  2. Due to splicing
• Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation?

  1. tRNA brings specific amino acid to ribosome
  2. Anticodon on tRNA binds to codon on mRNA
  3. Amino acids join by condensation reactions using ATP
• Describe how mRNA is produced from an exposed template strand of DNA?

  1. Free RNA nucleotides form complementary base pairs
  2. Phosphodiester bonds form
  3. By RNA polymerase
• Describe how mRNA is formed by transcription in eukaryotes?

  1. Hydrogen bonds between DNA bases break
  2. One DNA strand acts as a template
  3. Free RNA nucleotides align by complementary base pairing
  4. In RNA, uracil is used in place of thymine
  5. RNA polymerase joins adjacent RNA nucleotides, forms phosphodiester bonds
  6. Pre-mRNA is spliced to from mRNA
• Describe how a polypeptide is formed by translation of mRNA?

  1. mRNA attaches to ribosomes
  2. tRNA anticodons bind to complementary RNA codons
  3. tRNA brings a specific amino acid
  4. Amino acids join by peptide bonds, using ATP
  5. tRNA released
  6. The ribosome moves along the mRNA to form the polypeptide
• What is the proteome of a cell?

  1. Full number of different proteins that a cell is able to produce
• Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide. Do not include descriptions of transcription or splicing in the answer?

  1. mRNA associates with a ribosome
  2. Ribosome finds the start codon
  3. tRNA carries specific amino acid
  4. Anticodon on tRNA complementary to codon on mRNA
  5. Ribosome moves along to next codon
  6. Process repeated and amino acids join by peptide bonds
• Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer?

  1. mRNA binds to ribosome
  2. 2 codons/binding sites
  3. Allows tRNA with anticodons to bind
  4. Catalyses formation of peptide bonds between amino acids
• Define a mutagenic agent?

  A factor that increases the rate of mutation
• Apart from mutation, explain 1 other way genetic variation within a species is increased?

  1. Random fertilisation
  2. Produces new allele combinations
• Define 'gene mutation' and explain how a gene mutation can cause no effect or a positive effect?

  1. Change in base sequence of DNA results in the formation of new alleles
  2. No effect as genetic code is degenerate or mutation is in an intron
  3. Does change amino acid but no effect on tertiary structure
  4. Positive effect because results in change in polypeptide that positively changes the properties of a protein
  5. May result in increased survival chances
• Give 2 differences between mitosis and meiosis?

  1. 1 division in mitosis, 2 divisions in meiosis
  2. Crossing over only in meiosis
• Explain how the chromosome number is halved during meiosis?

  1. Homologous chromosomes pair
  2. One of each pair goes to opposite poles of the cell
• Describe the process of crossing over and explain how it increases genetic diversity?

  1. Homologous pairs of chromosomes associate
  2. Chiasmata form
  3. Alleles are exchanged
  4. Producing new combinations of alleles
• Explain why all the cells in a body can have the same mutation?

  1. Mutation that formed zygote
  2. All cells derived from zygote by mitosis