MCAT BIOLOGY EXAM -609 QUESTIONS AND ANSWERS 2024.

Central Dogma DNA → RNA → Protein Purines - which bases? - how many rings? Brainpower Read More 0:14 / 0:15 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 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 deoxygenaged (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 - temination 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 then degraded in nucleus. Exon Coding region of mRNA. Remains after activity of snRNPs and spliceosome. Then exits the nucleus for translation. DNA denaturation: conditions, effects Conditions: 1. Salt solution 2. High pH (basic) solution 3. High temperature Effects: - Double helix separates because hydrogen bonds are disrupted - DNA with more G-C pairs (G-C forms 3 H bonds, while A-T has only 2) has a greater Tm, so takes more energy to melt Restriction Enzymes - method bacteria use to protect themselves from viruses is to cut viral DNA w/these (bacterial DNA is methylated, so differentiated) - can form recombinant DNA (artificially) using these Cloning DNA Recombinant DNA can be placed in bacteria using a vector (plasmid or virus), then grown. Screened by including an antibiotic resistance gene and a lacZ gene to find which colonies actually took up the vector. cDNA Complementary DNA. Used often in cloning because has no introns (and therefore is active). Produced in RT PCR from mRNA. Southern Blot Identifies specific sequences of DNA through nucleic acid hybridization. Northern Blot Identifies specific sequences of RNA through nucleic acid hybridization. Western Blot Detects protein levels with antibodies. Genetic code: 3 major features 1. Degenerative: More than one codon for each amino acid 2. Unambiguous: Each codon encodes for only ONE amino acid 3. Universality: nearly every living organism uses same code Codon Three consecutive nucleotides on mRNA Stop codons UAA, UAG, UGA Start codon (and methoinine) AUG Convention for writing RNA 5' → 3' Translation Process of protein synthesis. Three steps: Initiation, elongation, termination. Translation (initiation) mRNA has left nucleus via nuclear pores, and is now in cytosol; initiation factors (proteins) help the 5' end of mRNA attach to the small ribosome subunit. tRNA with 5'-CAU-3' anticodon gets methionine and goes into P site. This together is called the "initiation complex." Ribosome - prokaryote - eukaryote In both prok and euk, made of rRNA and protein; small and large subunits. - prokaryote: 30S and 50S subunits (70S total) - eukaryote: 40S and 60S subunits (80S total); made in nucleolus; exported separately to the cytoplasm (prok. do not have nucleolus, but synthesis process is similar) Translation (elongation) tRNA with corresponding amino acid attaches to the A site (2 GTPs expended). Translocation occurs: ribosome shifts 3 nucleotide units along mRNA toward 3' end (1 more GTP expended), new tRNA now at P site. tRNA previously in P site moves to E (exits). Repeats until stop codon reaches P site.