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

Study Notes

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.

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 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.

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.

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.

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.

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.

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