3.4.1 DNA, genes and chromosomes

• In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.
• In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
• The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.
• A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA (including rRNA and tRNA).
• A gene occupies a fixed position on a chromosome called a locus.
• A sequence of 3 DNA bases is called a triplet and this codes for a specific amino acid.
• The genetic code is universal, meaning that the same triplet of bases codes for the same amino acid in all organisms.
• The genetic code is non-overlapping, meaning that each base is part of only one triplet.
• The genetic code is degenerate, meaning that more than one triplet code can code for the same amino acid
• In eukaryotes, there are non-coding regions in DNA called introns that don’t code for polypeptides.
• Exons are coding regions of DNA that code for amino acid sequences.
• Explain how change in the DNA base sequence for a protein may result in a change in the structure of the protein (3)

  • change in primary structure
  • change in hydrogen, disulfide or ionic bonds
  • alters tertiary structure
• Ribosomes are made up of RNA and proteins and are found in the cytoplasm.
• Gene diversity is the differences in DNA or differences in base sequences of DNA.
• A homologous pair of chromosomes are two chromosomes that carry the same genes and have the same gene loci, but different alleles.
• An allele is a different version of a gene that codes for a different amino acid.
• Adenine binds to thymine and guanine binds to cytosine in DNA, but in RNA adenine binds to uracil.
• Haploid cell has one copy of each chromosome (half of DNA), diploid cell has two copies of each chromosome.
• The genome is the entire set of genetic material in an organism, including all the genes and the DNA.
• An exon is a base sequence coding for a polypeptide.
• The genetic code uses four different DNA bases. What is the maximum number of different DNA triplets that can be made using these four bases ? (1)

  64
• Contrast the structures of DNA and mRNA molecules

  • DNA double stranded, mRNA single-stranded
  • DNA very long and RNA short
  • Thymine in DNA and uracil in RNA
  • deoxyribose in DNA and ribose in RNA
  • DNA has base pairing and mRNA does not
  • DNA has introns and mRNA does not
• Explain how the structure of DNA is related to it’s functions (6).

  • sugar-phosphate backbone so provides strength
  • long molecule so can store lots of information
  • helix so compact
  • base sequence allows information to be stored
  • double stranded so replication can occur semi-conservatively
  • weak hydrogen bonds for replication
• Describe how a gene is a code for the production of a polypeptide (3)

  • base sequence
  • in triplets
  • determine order/sequence of amino acid sequence
• Compare and contrast DNA in eukaryotic cells with the DNA in prokaryotic cells (5)

  Comparisons

  • nucleotide structure is identical
  • nucleotides joined by phosphodiester bond
  • DNA in mitochondria/chloroplasts similar structure to DNA in prokaryotes

  Contrasts

  • eukaryotic DNA is longer
  • eukaryotic DNA contain introns, prokaryotic DNA does not
  • eukaryotic DNA is linear, prokaryotic DNA is circular
  • eukaryotic DNA is associated with histones, prokaryotic DNA is not