AP Biology Unit 6: Mastering Molecular Genetics

Welcome to our exploration of DNA structure and replication!DNA has a unique double helix structure, composed of two strands that wind around each other.Each strand is made up of repeating units of sugar and phosphate groups, forming the backbone of DNA.The strands are connected by nitrogenous bases that pair in a specific way: A with T, and G with C.During DNA replication, the double helix is unwound by an enzyme called helicase.DNA polymerase adds new nucleotides to create the complementary strands.Primase creates RNA primers to start the process, while ligase joins the Okazaki fragments on the lagging strand.The leading strand is synthesized continuously in the direction of fork movement.The lagging strand is made in short segments called Okazaki fragments, which are later joined together.Now that we understand DNA structure and replication, let's move on to transcription.Transcription begins when RNA polymerase recognizes and binds to the promoter region of DNA.During initiation, RNA polymerase creates a transcription bubble, unwinding the DNA double helix.In the elongation phase, RNA polymerase moves along the template strand, adding complementary RNA nucleotides to create a growing RNA strand.After transcription, the pre-messenger RNA undergoes several modifications. First, a 5-prime cap is added to protect the RNA and assist with translation.A poly-A tail is added to the 3-prime end, which helps with RNA stability and export from the nucleus.Introns, which are non-coding regions, are removed through splicing, while exons are joined together to form the mature messenger RNA.Through alternative splicing, different combinations of exons can be joined together, allowing a single gene to produce multiple protein variants.Translation occurs at the ribosome, which consists of two subunits that come together around the messenger RNA.Translation begins at the start codon AUG, which codes for methionine. The initiator tRNA brings this first amino acid.During elongation, tRNAs bring amino acids matching each codon. The polypeptide chain grows one amino acid at a time.Translation ends when a stop codon is reached. The completed polypeptide chain is released from the ribosome.After release, the protein begins to fold into its functional three-dimensional structure.The protein may undergo post-translational modifications, where chemical groups are added to modify its function.Mutations in the genetic code can lead to changes in protein structure and function, potentially affecting cellular processes.