Test: DNA - 2 - MCAT MCQ

Telomeres are repetitive DNA sequences found at the ends of chromosomes. During DNA replication, the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. These fragments are then joined together by the enzyme DNA ligase. However, because the lagging strand synthesis starts away from the telomeres, a small portion of the telomeric DNA is not replicated during each round of DNA replication.

Over time, this loss of genetic material from the telomeres can lead to the shortening of the chromosomes. Telomerase, an enzyme that contains its own RNA template, can counteract this shortening by adding repetitive DNA sequences to the telomeres. This occurs during the enzymatic action of telomerase, which extends the telomeric DNA and maintains its length. However, telomerase activity is not present in all cell types and is particularly active in stem cells and certain cancer cells.

Therefore, while telomerase can help to counteract the loss of genetic material from telomeres, the actual loss occurs during the joining of adjacent Okazaki fragments.

In DNA and RNA, complementary base pairs are formed through hydrogen bonding between specific nucleotide bases. The bond dissociation energy refers to the energy required to break the hydrogen bonds between the base pairs.

In DNA, the complementary base pairs are A (adenine) with T (thymine) and G (guanine) with C (cytosine). In RNA, the base T is replaced by U (uracil).

Among the given options, the G:C base pair would require the most energy to break because it forms three hydrogen bonds, while the other base pairs form only two hydrogen bonds. The additional hydrogen bond in the G:C base pair increases the strength of the interaction between the bases, resulting in a higher bond dissociation energy.

Therefore, the association between G:C base pair would require the most energy to break compared to the other complementary base pairs.

The correct answer is C. Doxycycline would not affect human ribosomes because human ribosomes are made up of 60S and 40S subunits, while prokaryotic ribosomes are made up of 50S and 30S subunits.

Doxycycline specifically targets the bacterial ribosome and inhibits protein synthesis by binding to the 30S subunit in prokaryotes. The binding of doxycycline interferes with the bacterial ribosome's ability to read and translate mRNA, thereby inhibiting bacterial proliferation.

In eukaryotes, such as human cells, ribosomes have a different structure. They are composed of a 60S large subunit and a 40S small subunit. Since the structure of human ribosomes is different from prokaryotic ribosomes, doxycycline does not bind to or affect human ribosomes. Therefore, doxycycline is selective for inhibiting bacterial protein synthesis while having little effect on human cells.

Exons are the functional regions of DNA that contain coding sequences for protein synthesis. They are transcribed into mRNA and eventually translated into amino acids to form proteins. A point mutation within an exon can lead to a change in the coding sequence of the mRNA, which can result in the production of a protein with a different amino acid sequence.

In contrast, introns are non-coding regions of DNA that are transcribed into mRNA but are later removed through a process called splicing. Mutations within introns generally do not directly affect the amino acid sequence of the resulting protein.

Centromeres and telomeres are specialized DNA sequences involved in chromosome structure and stability, but they do not contain coding sequences for protein synthesis. Mutations in these regions are less likely to directly impact the amino acid sequence of proteins.

DNA polymerase is the enzyme responsible for DNA replication and DNA repair in living organisms. It catalyzes the addition of nucleotides to the growing DNA strand in the 5' to 3' direction. This means that the new DNA strand is synthesized by adding nucleotides to the 3' end of the growing strand.

Reverse transcriptase is an enzyme that catalyzes the synthesis of DNA from an RNA template. It is commonly found in retroviruses and is used in molecular biology techniques such as reverse transcription PCR (RT-PCR). Similar to DNA polymerase, reverse transcriptase also synthesizes DNA in the 5' to 3' direction, adding nucleotides to the 3' end of the growing DNA strand.

Therefore, both DNA polymerase and reverse transcriptase synthesize DNA in the 5' to 3' direction, which is the direction of DNA strand elongation.

The Golgi apparatus is an organelle involved in the modification, processing, and sorting of proteins and lipids in eukaryotic cells. One of its important functions is the glycosylation of proteins, which involves the attachment of sugar molecules to specific sites on proteins.

Defects in the Golgi apparatus and its glycosylation processes can lead to abnormalities in the structure and function of proteins. Abnormal patterns of glycosylation can affect the folding, stability, trafficking, and interactions of proteins, including regulatory proteins.

In the case described in the question, where the individual has a mysterious disease affecting connective tissues and defects in multiple regulatory proteins with abnormal glycosylation patterns, it suggests that the disease is most likely attributed to dysfunction or abnormalities in the Golgi apparatus. The Golgi apparatus plays a crucial role in the proper glycosylation of proteins, and abnormalities in this process can have significant consequences on protein function and ultimately lead to disease manifestations.

Pyrimidine dimers are a type of DNA damage that occurs when adjacent pyrimidine bases (thymine or cytosine) on the same DNA strand form covalent bonds with each other due to exposure to ultraviolet (UV) light. These dimers can distort the DNA structure and interfere with normal DNA replication and transcription.

To repair pyrimidine dimers, a class of enzymes called endonucleases is involved. Endonucleases are responsible for cleaving the damaged DNA strand at specific sites near the pyrimidine dimer. This creates a gap in the DNA molecule, allowing for the removal of the damaged section.

After the damaged DNA segment is removed, other DNA repair enzymes, such as DNA polymerases, helicases, and ligases, come into play to fill in the gap with new nucleotides and seal the DNA backbone, restoring the integrity of the DNA sequence.

However, specifically for the removal of pyrimidine dimers, it is the action of endonucleases that in

Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids during protein synthesis. The genetic code is degenerate, meaning that multiple codons can code for the same amino acid. This is due to the redundancy in the genetic code, where different combinations of the four nucleotides (A, U, G, and C) can specify the same amino acid.

For example, the amino acid leucine is specified by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. All of these codons code for the same amino acid, even though they have different nucleotide sequences.

This redundancy in the genetic code provides a level of error tolerance and allows for robustness in protein synthesis. It also allows for a more efficient use of the limited number of codons available in the genetic code.

tRNA binding sites are primarily located in the cytosol of eukaryotic cells. The cytosol is the fluid portion of the cytoplasm, which is the region between the cell membrane and the nucleus. It is the site where many cellular processes occur, including protein synthesis.

During protein synthesis, tRNA molecules bind to ribosomes in the cytosol and deliver specific amino acids to the growing polypeptide chain based on the codons on the mRNA template. The binding of tRNA to the ribosome occurs in the cytosol, where the actual process of translation takes place.

If a drug exclusively binds to tRNA binding sites, it would likely exert its effects in the cytosol by interfering with the binding of tRNA to ribosomes or by disrupting the translation process

Phosphodiester bonds are the covalent bonds that link nucleotides together in a DNA molecule. Each nucleotide consists of a phosphate group, a sugar (deoxyribose in the case of DNA), and a nitrogenous base (adenine, guanine, cytosine, or thymine in DNA). The phosphate group of one nucleotide forms a covalent bond with the sugar of the adjacent nucleotide, creating a backbone of alternating phosphate and sugar units.

The phosphodiester bond is formed through a condensation reaction, in which a molecule of water is released. The phosphate group of one nucleotide reacts with the hydroxyl group (-OH) on the 3' carbon of the sugar of the adjacent nucleotide, forming a phosphodiester bond and linking the two nucleotides together.

These covalent bonds between nucleotides create a stable backbone in DNA and contribute to the overall stability and integrity of the molecule. Hydrogen bonds, on the other hand, play a role in the pairing of complementary bases (A-T and G-C) between the two strands of DNA.