DNA, Genes & Chromosomes

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Prokaryotic DNA

  • Prokaryotic cells contain a single, circular DNA molecule that is not associated with proteins.
  • This circular DNA is sometimes called a nucleoid.
  • Prokaryotes often have additional small, circular DNA molecules called plasmids.
  • Plasmids contain a few genes and are more accessible for proteins needed for gene expression.
  • Plasmid genes are often required quickly, frequently, or in emergencies, such as antibiotic resistance genes.

Eukaryotic DNA

  • Eukaryotic DNA is linear and found in the nucleus, where it is very long.
  • DNA is associated with histone proteins (positively charged, globular) that help organise and condense it.
  • Other proteins associated with DNA include enzymes for DNA copying and repair.
  • The tightly coiled combination of DNA and proteins is called chromatin, which makes up chromatids and chromosomes.
  • Chromatids are organised into sections called genes.
  • The ends of chromatids are sealed with protective structures called telomeres.

Mitochondria & Chloroplast DNA

  • Mitochondria and chloroplasts contain their own DNA, called mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA).
  • This DNA is similar to prokaryotic DNA: it is short, circular, and not associated with proteins.
  • Mitochondrial DNA is located in the matrix of mitochondria.
  • Chloroplast DNA is located in the stroma of chloroplasts.

Genes

  • A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule.
  • Functional RNA molecules include mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
  • mRNA carries the base sequence used by ribosomes to form polypeptide chains.
  • tRNA carries amino acids to the ribosome during protein synthesis.
  • rRNA forms part of the structure of ribosomes.
  • The exact sequence of amino acids determines the shape and behaviour of a protein, so genes control protein structure and function.

Genetic Loci

  • The position of a gene on a chromosome is called its locus (plural: loci).
  • Each gene can exist in two or more different forms called alleles.
  • Different alleles of a gene have slightly different nucleotide sequences.
  • Alleles of the same gene always occupy the same locus on a chromosome.

The Triplet Code

  • The DNA nucleotide base code is a three-letter, or triplet, code; each triplet codes for one amino acid.
  • There are four bases (A, G, C, T), so there are 64 possible triplets (4³), but only 20 amino acids commonly occur in proteins.
  • Because multiple triplets can code for the same amino acid, the code is said to be degenerate; this can limit the effect of mutations.
  • Some triplets code for start (TAC – methionine) and stop signals, telling the cell where genes start and stop.
  • The code is non-overlapping, ensuring the cell reads the DNA correctly and produces the correct amino acid sequences.
  • The genetic code is universal: almost every organism uses the same code, so the same triplet codes for the same amino acid in all living organisms (with a few rare exceptions).

Non-Coding DNA

  • The eukaryotic genome contains many non-coding sections of DNA.
  • Non-coding DNA can be found between genes as non-coding multiple repeats (the same base sequences repeated multiple times).
  • Non-coding DNA can also be found within genes as introns, which sit between coding regions called exons.
  • During transcription, eukaryotic cells transcribe the whole gene (introns and exons) to produce pre-mRNA.
  • Before pre-mRNA exits the nucleus, introns are removed and exons are joined together in a process called splicing.

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練習問題

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  1. 1.Which of the following is a difference between prokaryotic and eukaryotic DNA?

    Easy
    • AProkaryotic DNA is circular; eukaryotic DNA is linear.
    • BProkaryotic DNA is linear; eukaryotic DNA is circular.
    • CProkaryotic DNA is associated with histones; eukaryotic DNA is not.
    • DProkaryotic DNA is longer than eukaryotic DNA.

  2. 2.Prokaryotic DNA is associated with histone proteins.

    Easy

    True or false?

  3. 3.Which type of DNA is found in the matrix of mitochondria?

    Medium
    • ANuclear DNA
    • BMitochondrial DNA
    • CChloroplast DNA
    • DPlasmid DNA

  4. 4.Which of the following are features of mitochondrial DNA? (select all that apply)

    Medium
    • AShort
    • BCircular
    • CAssociated with histones
    • DLinear
    • ENot associated with protein

  5. 5.Match the DNA type with its location.

    Medium
    • Mitochondrial DNA
    • Chloroplast DNA
    • Nuclear DNA
    • Mitochondrial matrix
    • Stroma
    • Nucleus

  6. 6.What is the term for the position of a gene on a chromosome?

    Easy
    • AAllele
    • BLocus
    • CChromatid
    • DTelomere

  7. 7.Which property of the genetic code is illustrated by the fact that the codons ACU and ACG both code for the amino acid threonine?

    Medium
    • AUniversal
    • BNon-overlapping
    • CDegenerate
    • DStart and stop signals

  8. 8.The genetic code is described as universal because almost every organism uses the same code.

    Easy

    True or false?

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