AQA Biology A-level: DNA, Genes, and Chromosomes

DNA, Genes, and Chromosomes

• DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) carry genetic information.
• DNA is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases.
• RNA is a relatively short polynucleotide chain.
• Nucleotides consist of a pentose sugar, a nitrogen-containing organic base, and a phosphate group.
• The components of a DNA nucleotide are deoxyribose, a phosphate group, and one of the organic bases adenine (A), cytosine (C), guanine (G), or thymine (T).
• Adenine and guanine are purine bases, while thymine, uracil, and cytosine are pyrimidines.
• Nucleotides join together by phosphodiester bonds formed in condensation reactions.

Genetic Code

• The order of bases on DNA is called the genetic code, which consists of triplets of bases, with each triplet (codon) coding for a particular amino acid.
• The amino acids are then joined together by peptide bonds to form a polypeptide chain.
• A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
• The location of a gene is called the locus.
• Not all the genome codes for proteins - non-coding sections of DNA are called introns, and the coding regions are called exons.
• There are four nucleotide bases that code for 20 different amino acids.

Protein Synthesis

• Protein synthesis involves two stages: transcription and translation.
• During transcription, a section of DNA is transcribed into mRNA, which is then translated into a polypeptide chain formed of amino acids.
• mRNA (messenger RNA) is a long single strand created during transcription, with a base sequence complementary to DNA.
• tRNA (transfer RNA) is a small molecule made up of around 80 nucleotides, with a clover-leaf shape.

Transcription

• During transcription, a molecule of mRNA is made in the nucleus:
  • Hydrogen bonds between complementary bases break, and DNA uncoils, separating the two strands and exposing the organic bases.
  • One of the DNA strands is used as a template to make the mRNA molecule (the template is called the antisense strand).
  • Free nucleotides line up by complementary base pairing, and adjacent nucleotides are joined by phosphodiester bonds, forming a molecule of mRNA.
  • The enzyme RNA polymerase catalyzes the formation of phosphodiester bonds.
  • When a stop codon is reached, transcription ceases.

Translation

• During translation, amino acids join together to form a polypeptide chain:
  • mRNA attaches to a ribosome, and tRNA collects amino acids from the cytoplasm and carries them to the ribosome.
  • tRNA attaches itself to mRNA by complementary base pairing - two molecules attach to mRNA at a time.
  • The amino acids attached to two tRNA molecules join by a peptide bond, and then the tRNA molecules detach themselves from the amino acids, leaving them behind.
  • This process is repeated, leading to the formation of a polypeptide chain until a stop codon is reached on mRNA, ending the process of protein synthesis.

Genetic Mutation

• Changes in DNA can arise spontaneously during replication, and any change in the base sequence or quantity of DNA is called a mutation.
• A change in the base sequence of a gene can change the sequence of amino acids.
• Sometimes, this can result in a mutation that is harmful, but due to the genetic code being degenerate, the amino acid sequence may not always be changed.

Evolution

• An environmental change occurs, and as a result, the selection pressure changes.
• Some individuals possess advantageous alleles, which give them a selective advantage and allow them to survive and reproduce.
• The advantageous alleles are passed on to their offspring, and over time, the frequency of alleles in a population changes, leading to evolution.

Types of Selection

• Directional Selection: occurs when environmental conditions change, and the phenotypes best suited to the new conditions are more likely to survive and breed.
• Stabilizing Selection: the phenotypes with successful characteristics are preserved, and those of greater diversity are reduced.

Impacts of Agriculture on Biodiversity

• Agricultural ecosystems reduce biodiversity and the number of species present because humans select for particular characteristics, reducing genetic diversity.
• Techniques to maintain biodiversity include:
  • Using hedgerows instead of fences
  • Growing different crops in the same area (intercropping)
  • Reducing the use of herbicides and pesticides
  • Preserving wetlands instead of draining them for farming use