2024/2025 MCAT Biology: Complete and Verified Exam Revision Questions with Detailed Answers for Exam Preparations

Central Dogma DNA → RNA → Protein Purines - which bases? - how many rings? Pyrimidines - which bases? - how many rings? Remember: "Pyrimidine" has a "y" in it; so does "cytosine" and "thymine." Phosphodiester Bond Joins one nucleotide to the next; between the 3rd C of one ribose and the 5th C of the other to create the sugar-phosphate backbone Directionality of DNA (convention) 5' → 3' (but strands are antiparallel, so the other strand is opposite) 3' of DNA attached to ... OH 5' of DNA attached to ... Phosphate group DNA composition 1. Phosphate group 2. 5-Carbon sugar 3. Nitrogenous base (A, T, G, C) Which nitrogenous bases form 2 hydrogen bonds? A, T Which nitrogenous bases form 3 hydrogen bonds? G, C Replisome Proteins that govern the replication process Origin of replication Where replication begins. Prokaryotes have 1; eukaryotes have many on each chromosome DNA synthesis: direction of synthesis 5' → 3' (the DNA is read 3' → 5') Steps of replication (5) 1. Helicase unzips double helix 2. RNA polymerase builds a primer 3. DNA polymerase adds leading/lagging strands 4. Primers removed 5. Okazaki fragments joined RNA vs. DNA: differences RNA: - C2 is oxygenated (has OH) - Single stranded - Uses uracil instead of thymine - Can move through nuclear pores out of nucleus DNA - C2 is deoxygenated (has H) - Double stranded (double helix) - Uses thymine - Stuck in the nucleus Transcription Process of making RNA (rRNA, mRNA, tRNA) Promoter Required for transcription. Sequence of DNA nucleotides that signals beginning point for transcription. Primer Required for DNA replication Consensus sequence Most common promoter sequences; closer the DNA nucleotides are to the consensus sequence, the more tightly the RNA polymerase can bind, which leads to more frequent transcription (and vice versa) RNA polymerase Synthesizes RNA in transcription Transcription: Steps 1. Initiation - transcription factors, transcription initiation complex (including RNA pol) finds promoter 2. Elongation - Template/antisense DNA strand is read and complementary RNA synthesized in 5' → 3' direction (same as DNA synthesis); DNA is read in 3' → 5' (also same as DNA synthesis) 3. Termination - termination sequence marks end, special proteins dissociate RNA pol from DNA Gene regulation Most occurs at transcription via repressors and activators, which bind near promoter and affect activity of RNA polymerase Operon Entire transcript in a prokaryote; includes multiple genes (polycistrionic). I.e., lac operon - operator, repressor, genes, promoter, etc. RNA post-transcriptional processing pre-mRNA altered in 3 ways: 1. addition of nucleotides 2. deletion of nucleotides 3. modification of nitrogenous bases 5' cap Added to mRNA as an attachment site in protein synthesis and protection against degradation by exonucleases. Done with GTP. 3' poly A tail Added to mRNA to protect from exonucleases. snRNPs Recognize introns and snip them out Intron Non-coding region of mRNA. Is removed by snRNPs (and spliceosome complexes) and