DNA Replication, Polypeptide Synthesis & Mutations

DNA Replication

  • Outline how the Watson-Crick model of DNA explains the exact replication of DNA
    • The Watson-Crick model of DNA describes the structure of DNA as a double helix made up of two complementary strands of nucleotides.
    • DNA replication is the process by which a cell makes a copy of its DNA prior to cell division.
    • DNA replication starts at specific points in the DNA molecule, known as origins of replication, where an enzyme called helicase unwinds the DNA double helix, separating the two strands.
    • Each strand of DNA serves as a template for the synthesis of a new complementary strand, with the enzyme DNA polymerase adding nucleotides in a sequence that is complementary to the template strand.
    • The hydrogen bonds between the nitrogenous bases ensure that the new nucleotides are added in the correct sequence.
    • Once the new strands have been synthesized, an enzyme called DNA ligase joins the fragments of DNA together, creating a continuous strand.
    • Errors in DNA replication can occur due to mutagens or accidental mistakes, which can lead to genetic variation and evolution.

  Step-by-step DNA replication:

  

  1. DNA replication starts at specific points in the DNA molecule, known as origins of replication.
  2. An enzyme called helicase unwinds and separates the two strands of the DNA double helix.
  3. Each strand of DNA serves as a template for the synthesis of a new complementary strand.
  4. The enzyme DNA polymerase adds nucleotides to the growing strand in a sequence that is complementary to the template strand.
  5. The hydrogen bonds between the nitrogenous bases ensure that the new nucleotides are added in the correct sequence.
  6. Once the new strands have been synthesized, an enzyme called DNA ligase joins the fragments of DNA together, creating a continuous strand.

  DNA replication is essential for the growth and division of cells, and it ensures that the genetic information is passed on accurately to the daughter cells.

Polypeptide Synthesis

Transcription:

  1. DNA unwinds and unzips at the specific region to be transcribed by the enzyme helicase.
  2. Primase adds short RNA primers to the template DNA strand to start the process of transcription.
  3. RNA polymerase binds to the DNA template strand and fetches the free-floating nucleotides from the cytoplasm.
  4. RNA polymerase adds nucleotides to the growing mRNA molecule in a sequence complementary to the DNA template strand.
  5. Once the mRNA molecule is complete, it is processed by enzymes and transported out of the nucleus into the cytoplasm.

Translation:

  1. The mRNA molecule carries the genetic code for the polypeptide chain to be synthesized.
  2. Ribosomes read the mRNA sequence and translate each three-nucleotide sequence (codon) into a specific amino acid.
  3. Transfer RNA (tRNA) molecules carry the corresponding amino acids to the ribosome, where they bind to the codon on the mRNA molecule through their anticodon sequence.
  4. The ribosome links the amino acids together, forming a polypeptide chain in a sequence determined by the mRNA sequence.
  5. The polypeptide chain folds into a specific three-dimensional structure to form a functional protein.

In summary, during transcription, DNA unwinds and unzips, primers are added to the DNA template strand, and RNA polymerase adds nucleotides to the growing mRNA molecule in a sequence complementary to the DNA template strand. Once the mRNA molecule is complete, it is processed by enzymes and transported out of the nucleus into the cytoplasm. During translation, the mRNA molecule is translated into a specific sequence of amino acids by ribosomes, and the transfer RNA (tRNA) molecules carry the corresponding amino acids to the ribosome, where they bind to the codon on the mRNA molecule through their anticodon sequence. The polypeptide chain folds into a specific three-dimensional structure to form a functional protein.

Mutations

Define the term ‘mutation’

  • A mutation is a change in the DNA sequence of an organism's genome, which can occur due to various factors and can have various effects on an organism's phenotype.

State some factors which can cause a mutation

  • Errors during DNA replication: Mistakes can occur during DNA replication, leading to changes in the DNA sequence.
  • Exposure to mutagens: Chemicals, radiation, and other environmental factors can damage DNA and cause mutations.
  • Spontaneous mutations: Some mutations occur spontaneously without any known cause.
  • Errors in DNA repair mechanisms: Cells have mechanisms in place to repair DNA damage, but if these mechanisms fail or make mistakes, mutations can occur.
  • Viral infections: Some viruses can integrate their genetic material into the host cell's DNA, potentially causing mutations or other genetic changes.

Point mutation

  • A point mutation is a change in a single nucleotide in a DNA sequence.
  • Examples of point mutations include substitutions, insertions, and deletions.
  • Point mutations can affect the function of a single gene and may or may not have a significant effect on an organism's phenotype.
Types of point mutations
  1. Substitution:
  • A substitution is a type of point mutation that involves the replacement of one nucleotide in a DNA sequence with another nucleotide.
  • Substitutions can be classified as synonymous or non-synonymous.
  • Synonymous substitutions do not change the amino acid sequence of the protein, while non-synonymous substitutions can result in a different amino acid being incorporated into the protein, which can affect the protein's structure and function.
  1. Insertion:
  • An insertion is a type of point mutation that involves the addition of one or more nucleotides to a DNA sequence.
  • Insertions can cause a frameshift mutation, in which the reading frame of the DNA sequence is shifted, potentially changing the amino acid sequence of the resulting protein.
  1. Deletion:
  • A deletion is a type of point mutation that involves the removal of one or more nucleotides from a DNA sequence.
  • Deletions can also cause a frameshift mutation, potentially changing the amino acid sequence of the resulting protein.
Consequences of point mutations
  1. Silent Mutation:
  • A silent mutation is a type of substitution that does not result in a change in the amino acid sequence of the protein.
  • Since the genetic code is degenerate (meaning that more than one codon can code for the same amino acid), some substitutions can occur without changing the protein that is produced.
  • Silent mutations are usually considered neutral, as they do not affect the function of the protein.
  1. Missense Mutation:
  • A missense mutation is a type of substitution that results in a change in the amino acid sequence of the protein.
  • Depending on the location and nature of the substitution, missense mutations can have a range of effects on the protein's structure and function, which can affect the organism's phenotype.
  • Missense mutations can be neutral, deleterious, or even beneficial if they confer a new function on the protein.
  1. Nonsense Mutation:
  • A nonsense mutation is a type of substitution that results in the creation of a premature stop codon in the DNA sequence.
  • This premature stop codon truncates the protein, leading to a nonfunctional or truncated protein.
  • Nonsense mutations are usually deleterious and can cause genetic diseases.
  1. Frameshift Mutation:
  • A frameshift mutation is a type of insertion or deletion that causes a shift in the reading frame of the DNA sequence.
  • This shift can change the amino acid sequence of the protein downstream of the mutation, leading to a nonfunctional or truncated protein.
  • Frameshift mutations are usually deleterious and can cause genetic diseases.

Chromosomal mutation

Chromosomal mutations are changes in the structure or number of chromosomes.

  • A chromosomal mutation is a change in the structure or number of chromosomes in a cell.
  • Examples of chromosomal mutations include deletions, duplications, inversions, translocations, and aneuploidy.
  • Chromosomal mutations can affect a large number of genes and may have significant effects on an organism's phenotype.
Comparison between the two types of mutations
  • Chromosomal mutations involve changes in the structure or number of chromosomes, while point mutations involve changes in individual nucleotides in DNA sequences.
  • Chromosomal mutations can affect a large number of genes and have a significant effect on an organism's phenotype, while point mutations may affect the function of a single gene and may or may not have a significant effect on an organism's phenotype.
  • Chromosomal mutations are often caused by errors during meiosis or exposure to mutagens, while point mutations can be caused by errors during DNA replication, exposure to mutagens, or spontaneous changes in DNA sequence.
  • Both types of mutations can be harmful, beneficial, or have no effect on an organism's fitness, depending on the specific changes that occur and the context in which they occur.