MCAT Biology Test Questions and Answers 2023 Latest Update, Exams of Nursing

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Questions and Answers

2023 Latest Update.

Central Dogma - Answer DNA → RNA → Protein Purines

• which bases?
• how many rings? - Answer Pyrimidines
• which bases?
• how many rings? - Answer Remember: "Pyrimidine" has a "y" in it; so does "cytosine" and "thymine." Phosphodiester Bond - Answer 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

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2023 Latest Update.

Directionality of DNA (convention) - Answer 5' → 3' (but strands are antiparallel, so other strand is opposite) 3' of DNA attached to ... - Answer OH 5' of DNA attached to ... - Answer Phosphate group DNA composition - Answer 1. Phosphate group

2. 5-Carbon sugar
3. Nitrogenous base (A, T, G, C) Which nitrogenous bases form 2 hydrogen bonds? - Answer A, T

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2023 Latest Update.

Which nitrogenous bases form 3 hydrogen bonds? - Answer G, C Replisome - Answer Proteins that govern the replication process Origin of replication - Answer Where replication begins. Prokaryotes have 1; eukaryotes have many on each chromosome DNA synthesis: direction of synthesis - Answer 5' → 3' (the DNA is read 3' → 5') Steps of replication (5) - Answer 1. Helicase unzips double helix

2. RNA polymerase builds a primer

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2023 Latest Update.

3. DNA polymerase adds leading/lagging strands
4. Primers removed
5. Okazaki fragments joined RNA vs. DNA: differences - Answer 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)

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2023 Latest Update.

• Uses thymine
• Stuck in the nucleus Transcription - Answer Process of making RNA (rRNA, mRNA, tRNA) Promoter - Answer Required for transcription. Sequence of DNA nucleotides that signals beginning point for transcription. Primer - Answer Required for DNA replication Consensus sequence - Answer 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)

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2023 Latest Update.

RNA polymerase - Answer Synthesizes RNA in transcription Transcription: Steps - Answer 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 - Answer Most occurs at transcription via repressors and activators, which bind near promoter and affect activity of RNA polymerase

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2023 Latest Update.

Operon - Answer Entire transcript in a prokaryote; includes multiple genes (polycistrionic). I.e., lac operon - operator, repressor, genes, promoter, etc. RNA post-transcriptional processing - Answer pre-mRNA altered in 3 ways:

1. addition of nucleotides
2. deletion of nucleotides
3. modification of nitrogenous bases 5' cap - Answer Added to mRNA as an attachment site in protein synthesis and protection against degradation by exonucleases. Done with GTP.

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2023 Latest Update.

3' poly A tail - Answer Added to mRNA to protect from exonucleases. snRNPs - Answer Recognize introns and snip them out Intron - Answer Non-coding region of mRNA. Is removed by snRNPs (and spliceosome complexes) and then degraded in nucleus. Exon - Answer Coding region of mRNA. Remains after activity of snRNPs and spliceosome. Then exits the nucleus for translation. DNA denaturation: conditions, effects - Answer Conditions:

1. Salt solution
2. High pH (basic) solution
3. High temperature

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2023 Latest Update.

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 - Answer - 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 - Answer 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.

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2023 Latest Update.

cDNA - Answer Complementary DNA. Used often in cloning because has no introns (and therefore is active). Produced in RT PCR from mRNA. Southern Blot - Answer Identifies specific sequences of DNA through nucleic acid hybridization. Northern Blot - Answer Identifies specific sequences of RNA through nucleic acid hybridization. Western Blot - Answer Detects protein levels with antibodies. Genetic code: 3 major features - Answer 1. Degenerative: More than one codon for each amino acid

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2023 Latest Update.

2. Unambiguous: Each codon encodes for only ONE amino acid
3. Universality: nearly every living organism uses same code Codon - Answer Three consecutive nucleotides on mRNA Stop codons - Answer UAA, UAG, UGA Start codon (and methoinine) - Answer AUG Convention for writing RNA - Answer 5' → 3' Translation - Answer Process of protein synthesis. Three steps: Initiation, elongation, termination.

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2023 Latest Update.

Translation (initiation) - Answer 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 - Answer 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)

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2023 Latest Update.

Translation (elongation) - Answer 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. A site - Answer Aminoacyl site of translation

• site where tRNA bearing amino acid first attaches to ribosome P site - Answer Peptidyl site of translation
• site where first tRNA bearing methionine goes
• site where amino acid chain is elongated

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2023 Latest Update.

E site - Answer Exit site of translation

• tRNA (now w/o amino acid) leaves the ribosomal complex Translocation (translation)
• purpose
• energy expenditure - Answer Translocation is a step of elongation; the ribosome shifts 3 nucleotide units toward 3' end of mRNA so that the next codon is exposed. 3 GTPs are used in total. Termination (translation) - Answer Translation ends when a stop codon reaches the P site; release factor protein binds to stop codon and ribosom disassembles.

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2023 Latest Update.

Post-translational modifications of peptides - Answer - Other groups may be added to the peptide chain (sugars, lipids, phosphates)

• May be cleaved in one or more places Where does translation take place? - Answer Two possibilities:

1. Free-floating ribosome in the cytosol
2. Ribosome that attaches itself to the rough ER during translation and injects proteins into ER lumen How / why do some ribosomes attach to rough ER? - Answer A 20- amino acid sequence (signal peptide) near front of polypeptide is recognized by protein-RNA signal-recognition particle (SRP) that carries ribosome complex to a receptor protein on the ER.

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2023 Latest Update.

Gene mutation - Answer Alteration in the sequence of DNA in a single gene Chromosomal mutation - Answer The structure of a chromosome is changed Mutagens - Answer Chemical or physical agents that can cause mutations (induced) Point mutation - Answer A single base-pair of nucleotides in a double strand of DNA is changed Base-pair substitution mutation - Answer Type of point mutation. One base pair is replaced by another.

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2023 Latest Update.

Missense mutation - Answer Base-pair mutation that occurs in amino acid coding sequence of a gene. Does not always affect the a.a. or the protein:

• No change in protein function: neutral mutation
• amino acid is not changed: silent mutation Insertion / Deletion - Answer Another type of point mutation. May result in a frameshift mutation if occurs in multiples other than 3. Frameshift mutation - Answer Occurs when insertion / deletion occurs in multiples other than 3. Bases are then grouped incorrectly, leading to incorrect "reading." Usually results in nonfunctional proteins.

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2023 Latest Update.

Nonsense mutation - Answer Premature stop codon due to base-pair substitution, insertion, or deletion. VERY serious. Chromosomal deletion - Answer DNA fragment breaks off from a chromosome Chromosomal duplication - Answer DNA fragment breaks off one chromosome and attaches to its homologous chromosome Chromosomal translocation - Answer Segment of DNA is inserted into another chromosome Chromsomal inversion - Answer Orientation of a section of DNA on a chromosome is reversed

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2023 Latest Update.

Causes of translocation / inversion of chromosomes (possible) - Answer Transposition: transposons/transposable elements excise themselves and reinsert elsewhere. This is one way a somatic cell can alter genetic makeup w/o meiosis. Forward mutation - Answer Mutating an organism farther from its wild type state Backward mutation - Answer Mutating an organism back toward its wild type state (from a mutated state) Chromatin - Answer Coiled DNA/protein complex.

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2023 Latest Update.

Histones - Answer Globular proteins around which DNA is wrapped to form chromatin. Nucleosome - Answer Eight histones + DNA What is the charge of histones? - Answer They are basic, and have a net positive charge at the normal pH of a cell. When is chromatin coiled/uncoiled? - Answer Coiled: nuclear division Uncoiled: transcription (euchromatin) Homologous chromosomes (homologues) - Answer Two chromosomes that encode for the same traits

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2023 Latest Update.

Cell life cycle: steps (diagram) - Answer G₁, S, G₂, M Cell life cycle: Interphase - Answer G₁ (first growth phase) S (synthesis) G₂ (second growth phase) Cell life cycle: G₁ (characteristics) - Answer - cell has just split

• new organelles, proteins produced = growth
• G₁ checkpoint: appropriate ratio of cytoplasm to DNA = cell enters S (division) phase; otherwise goes to G₀
• normal longest stage of cycle

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2023 Latest Update.

Cell life cycle: G₀ (characteristics) - Answer - nongrowing phase (NOT interphase)

• allows for differences in length of cell cycle; i.e., mature neurons permanently in G₀ Cell life cycle: S (characteristics) - Answer - cell's energy devoted to replicating DNA
• technically maintains same chromosome number, but now each chromosome has identical sister chromatid Cell life cycle: G₂ (characteristics) - Answer - cell prepares to divide

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2023 Latest Update.

• G₂ checkpoint: when there is enough mitosis promoting factor (MPF), mitosis is triggered Mitosis: steps - Answer Prophase Metaphase Anaphase Telophase (Remember: PMAT) Prophase: characteristics (mitosis) - Answer - condensation of chromatin into chromosomes
• centrioles move to opposide ends of cells
• nucleolus and nucleus disappear

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2023 Latest Update.

• spindle apparatus forms Spindle apparatus - Answer - aster fibers (microtubules radiating from centrioles)
• kinetochore microtubules growing from centromeres of chromosomes
• spindle microtubules connecting centrioles Metaphase: characteristics (mitosis) - Answer Chromosomes align at the equator of the cell Anaphase: characteristics (mitosis) - Answer - sister chromatids split at centromeres and move to opposite ends of cells
• cytokinesis may start toward end of anaphase

Questions and Answers

2023 Latest Update.

Telophase: characteristics (mitosis) - Answer - nuclear membrane reforms

• nucleolus reforms
• chromosomes decondense
• cytokinesis continues Meiosis: definition - Answer Double nuclear division that produces four haploid (n) gametes from one (2n) cell. Which cells undergo meiosis?

1. male
2. female - Answer 1) male: spermatogonium (spermatogenesis)