Step 1 First Aid - Biochemistry Questions with Detailed Verified Answers (100% Correct Answers) /Already Graded A+

1 Exampromax - Stuvia US 2025/2026


Step 1 First Aid - Biochemistry Questions with Detailed
Verified Answers (100% Correct Answers) /Already
Graded A+
Chromatin structure
Ans: Negatively charged DNA loops twice around histone octamer (2 each
of the positively charged H2A, H2B, H3, and H4) to form nucleosome
bead. H1 ties nucleosomes together in a string. (Think of "beads on a
string"; H1 is the only histone that is not in the nucleosome core.) In
mitosis, DNA condenses to form mitotic chromosomes. <img src="66a -
Euchramatin strxr.JPG" />

Heterochromatin
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Ans: Condensed, transcriptionally inactive ("H eteroC hromatin = H ighly
C ondensed.")
Approved By:




Euchromatin
Ans: Less condensed, transcriptionally active (Eu = true, "truly
transcribed")

Purines
Ans: A, G 2 Rings ("PUR e A s G old = PUR ines") <img src="67b -
Purine strxr.JPG" />

Pyrimidines
Ans: C, T, U 1 ring ("CUT the PY (pie): PY rimidines") <img src="67c -
Pyrimidine strxr.JPG" />

Functional groups of the nucleosides
Ans: Guanine has a ketone. Thy mine has a methy l. Deamination of
cytosine makes uracil.

Base differences btw RNA and DNA

, 2 Exampromax - Stuvia US 2025/2026

Ans: Uracil is found in RNA; Thymine in DNA

Base pair bonds
Ans: G-C bond (3 H-bonds) is stronger than A-T bond (2 H-bonds). Incr G-
C content --< higher melting temperature.

AA's necessary for purine synthesis
Ans: G lycine A spartate G lutamine

Nucleoside
Ans: Base + ribose

Nucleotide
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Ans: Base + ribose + phosphate; linked by 3'-5' phosphodiester bond.
Approved By:




Purines are made from...?
Ans: IMP precursor (see bottom/right) <img src="68a - Nucleotide
synthesis.JPG" />

Pyrimidines are made from...?
Ans: Orotate precursor, with PRPP added later. <img src="68a -
Nucleotide synthesis.JPG" />

Deoxyribonucleotide synthesis
Ans: Ribonucleotides are synthesized first and are converted to
deoxyribonucleotides by ribonucleotide reductase. <img src="68a -
Nucleotide synthesis.JPG" />

ABX and anti-neoplastic drugs that function by interfering w/ nucleotide
synthesis (list)
Ans: Hydroxyurea 6-mercaptopurine 5-fluorouracil Methotrexate
Trimethoprim

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Hydroxyurea
Ans: Inhibits ribonucleotide reductase. <img src="68a - Nucleotide
synthesis.JPG" />

6-mercaptopurine (6-MP)
Ans: Blocks de novo purine synthesis. <img src="68a - Nucleotide
synthesis.JPG" />

5-Fluorouracil (5-FU)
Ans: Inhibits thymidilate synthase (decr dTMP). <img src="68a -
Nucleotide synthesis.JPG" />

Methotrexate
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Ans: Inhibits dihydrofolate reductase (decr dTMP) <img src="68a -
Approved By:




Nucleotide synthesis.JPG" />

Trimethoprim
Ans: Inhibits bacterial dihydrofolate reductase (decr dTMP) <img src="68a
- Nucleotide synthesis.JPG" />

Transition vs. transversion
Ans: Transition: Substituting purine for purine or pyrimidine for
pyrimidine ("TransI tion = I dentical type") Transversion: Substituting
purine for pyrimidine or vice versa ("TransV ersion = conV ersion btw
types")

Genetic code: unambiguous
Ans: Each codon specifies only 1 AA.

Genetic code: degenerate/redundant
Ans: < 1 codon may code for the same AA. (Methionine is encoded by 1
codon: AUG)

Genetic code: Commaless, nonoverlapping

, 4 Exampromax - Stuvia US 2025/2026

Ans: Read from a fixed starting point as a continuous sequence of bases.
*some viruses are an exception.

Genetic code: universal
Ans: Genetic code is conserved throughout evolution. *exceptions include
mitochondria, archaebacteria, Mycoplasma , and some yeasts

Silent mutation
Ans: Same AA, often base change in 3rd position of codon (tRNA wobble)

Missense mutation
Ans: Changed AA (conservative -- new AA is similar in chemical structure)
Exampromax - Stuvia US




Nonsense mutation
Approved By:




Ans: Change resulting in early stop codon ("Stop the nonsense !")

Frame shift mutation
Ans: Change resulting in misreading of all nucleotides downstream,
usually resulting in a truncated, nonfunctional protein.

Severity of damage in DNA mutations
Ans: Nonsense < missense < silent

Eukaryotic vs. prokaryotic DNA replication.
Ans: Eukaryotic DNA replciation is more complex, but uses many
analogous enzymes. In both: DNA replication is semiconservative and
involves both continuous and discontinuous (Okazaki fragment) synthesis.
For eukaryotes, replication begins at a consensus sequence of base pairs.

Origin of replication
Ans: Particular sequence in genome where DNA replication begins. May
be single (prokaryotes) or multiple (eukaryotes).