Genetic information official content revision notes

Genetic Information

Genes and DNA

• Genes are segments of DNA that code for polypeptides or functional RNA. Each gene consists of a specific sequence of nucleotide bases.
• Alleles are different versions of a gene that arise due to mutations.
• Chromosomes are structures made of DNA and proteins that contain many genes. In humans, chromosomes are organized into pairs.
• The genome is the complete set of genes in an organism, while the proteome refers to the entire set of proteins expressed by the genome.

DNA Structure

• In eukaryotic cells, DNA is associated with histone proteins, forming a complex that helps in packaging DNA into chromosomes.
• Prokaryotic cells have a simpler structure, with circular DNA located in the nucleoid region and plasmids that may carry additional genes.

The Genetic Code

• The genetic code is triplet, meaning three nucleotide bases code for one amino acid. It is also degenerate, as multiple codons can code for the same amino acid, and universal, as it is largely the same across all organisms.

Transcription and Translation

• Transcription is the process where DNA is transcribed to form pre-mRNA, which undergoes processing to become mature mRNA.
• Translation involves the decoding of mRNA by ribosomes, using tRNA to bring the appropriate amino acids to form polypeptides.

Mutations and Variation

• Mutations can occur as base substitutions or deletions, leading to changes in the protein produced.
• Chromosome non-disjunction during meiosis can lead to aneuploidy, affecting genetic diversity.
• Meiosis introduces variation through independent segregation and crossing over.

Natural Selection

• Natural selection drives allele frequency changes over generations, influenced by environmental pressures. Directional and stabilizing selection are two types of selection that can affect populations differently.