Molecular Genetics — Flashcards

Flashcard 381: How do the following mutations affect protein speed in gel electrophoresis compared to normal Hemoglobin (HbA)? - Frameshift mutation - Missense mutation - Nonsense mutation - Silent mutation - Trinucleotide expansion

Answer: - **Frameshift mutation:** Can result in shorter or longer proteins. In α-thalassemia, loss of α-chains leads to formation of **HbH (β4)**, which moves **faster** than HbA toward the anode. - **Missense mutation:** Single amino acid substitution (e.g., HbS, HbC). These lose negative charge compared to HbA and thus move **slower** toward the anode (HbA > HbS > HbC). - **Nonsense mutation:** Results in a truncated (shorter) protein, which moves **faster** due to reduced size/mass. - **Silent mutation:** No change in amino acid sequence; mobility remains the **same** as HbA. - **Trinucleotide expansion:** Results in a larger protein (e.g., Huntington disease), which moves **slower** due to increased size.

Flashcard 382: Match the following with their specific definition? -Unwinding of double helix -Removal of supercoils -Stabilization of unwound template strands -Synthesis of RNA primer -5' to 3 DNA synthesis & 3' to 5' exonuclease activity -Removes RNA primer -Has 5' to 3' exonuclease activity -Joining of Okazaki fragments -Similar work to DNA polymerase 3 -Connect the lagging strands together.

Answer: Unwinding of double helix: Helicase -Removal of supercoils: DNA gyrase (Topoisomerase 2) -Stabilization of unwound template strands: single stranded DNA binding protein -Synthesis of RNA primer: Primase (RNA polymerase) -5' to 3 DNA synthesis & 3' to 5' exonuclease activity: DNA polymerase 3 -Removes RNA primer: DNA polymerase 1 -Has 5' to 3' exonuclease activity: DNA polymerase 1 -Joining of Okazaki fragments: DNA ligase -Similar work to DNA polymerase 3: DNA polymerase 1, & 2 -Connect the lagging strands together: DNA ligase

Flashcard 383: Describe the characteristics or mutations associated with the following Hemoglobins: - Hemoglobin C - Hemoglobin F - Hemoglobin H - Hemoglobin S

Answer: - **HbC**: Glutamate → **Lysine** mutation at position 6 of the β-globin chain. - **HbF**: Fetal hemoglobin (α2γ2); has higher O2 affinity than HbA. - **HbH**: β4 tetramers; occurs in **3-gene deletion** α-thalassemia. - **HbS**: Glutamate → **Valine** mutation at position 6 of the β-globin chain.

Extra: Note: **Hb Barts** (γ4) occurs in **4-gene deletion** α-thalassemia (Hydrops fetalis). Both HbH and Hb Barts have very high oxygen affinity and are poor at oxygen delivery. Mnemonic for β6 mutations: - Hb**S**: **V**aline (**S**ickle) - Hb**C**: **L**ysine (Ly-**C**-ine)

Flashcard 384: What are the following processes or structures associated with? - Protein degradation - mRNA splicing

Answer: 1. Protein degradation: **Proteasomes** (ubiquitin-mediated degradation) 2. mRNA splicing: **Spliceosomes** (snRNPs)

Extra: Proteasomes degrade nuclear and cytoplasmic proteins tagged with ubiquitin. Spliceosomes consist of snRNPs and other proteins that remove introns from pre-mRNA. Clinical correlate: Anti-Smith antibodies (highly specific for SLE) are directed against snRNPs. Anti-U1 RNP antibodies are associated with Mixed Connective Tissue Disease (MCTD).

Flashcard 385: Identify the location or function of the following molecular markers: - CCA sequence - Poly-A tail - 7-methylguanosine cap - TATA box

Answer: - **CCA sequence**: Located at the 3' end of **tRNA**; the site where amino acids bind ("**C**an **C**arry **A**mino acids"). - **Poly-A tail**: Added to the 3' end of **mRNA** after transcription (post-transcriptional modification). - **7-methylguanosine cap**: Located at the 5' end of **mRNA** (post-transcriptional modification). - **TATA box**: An upstream eukaryotic **DNA promoter** sequence that binds TATA-binding protein (TBP) to initiate transcription.

Flashcard 386: What exonuclease activities are present in DNA polymerase I and DNA polymerase III?

Answer: DNA Polymerase I: Both 5'→3' and 3'→5' exonuclease activities DNA Polymerase III: 3'→5' exonuclease activity only

Flashcard 387: In an experiment, individual bacterial colonies lack a specific enzyme involved in DNA replication. - This enzyme is responsible for removing short fragments of RNA that are base-paired to the DNA template. - Which enzyme is most likely deficient in this strain of E. coli?

Answer: DNA polymerase I It removes RNA primers (5' to 3' exonuclease activity) and replaces them with DNA nucleotides. **Note:** Do not confuse with **Primase**, which synthesizes the RNA primers.

Flashcard 388: In which strands will the followings occurs? (leading or lagging strands) -Enzymatic function of DNA helicase -Interaction with single stranded DNA binding proteins -Joining of DNA fragments by ligase -Proofreading of the newly synthesized DNA -Relief of supercoils by topoisomerase

Answer: They all will occur in both leading and lagging strands except for Joining of DNA fragments by ligase which only occurs in the lagging strands.

Flashcard 389: What are the two types of DNA mutation that occurs in duchenne muscular dystrophy?

Answer: Frameshift, nonsense mutation.

Flashcard 390: True or False in relation to sickle cell anemia? a.) HbS decreases Beta -globin interaction with 2,3-diphosphoglycerate b.) HbS impairs oxygen binding to the heme moiety c.) HbS allows hydrophobic interaction among hemoglobin molecules.

Answer: a.) **False**: The mutation (Glu → Val) is on the surface of the $\beta$-globin chain and does not significantly alter the 2,3-BPG binding site. b.) **False**: HbS molecules bind oxygen normally; the pathology is the polymerization of *deoxygenated* HbS. c.) **True**: Replacing polar Glutamate with non-polar Valine creates a hydrophobic "patch" on the surface, allowing interaction with other hemoglobin molecules.

Extra: In **Sickle Cell Anemia**, a point mutation in the $\beta$-globin gene (GAG $\to$ GTG) results in the substitution of **Glutamate (polar)** with **Valine (non-polar)** at the 6th position. This hydrophobic Valine residue fits into a hydrophobic pocket on an adjacent hemoglobin molecule (specifically the $\beta$ chain of another deoxy-HbS tetramer), leading to **polymerization** and the characteristic "sickling" of the RBC.