Test on DNA, Chromosomes, and Genomes

Question 1: Approximately 1% of the human genome is composed of sequences that code for proteins.

A. Ano

B. Ne

Explanation: The study materials state that 'only a small part, about 1% of the genome, codes for proteins'.

Question 2: Which statement accurately describes the evolutionary relationship and chromosomal changes between mouse and human genomes according to the provided materials?

A. Human chromosome 14 and mouse chromosome 12 show conserved synteny, meaning they have largely identical genes in the same order, despite 180 breakage-and-rejoining events since their last common ancestor.

B. Despite evolving over 100 million years, the mouse and human genomes show identical DNA content, with no net loss or gain of DNA in either lineage.

C. Approximately 50% of the human genome is composed of syntenic regions that are perfectly matched with the mouse genome, indicating minimal evolutionary changes.

D. The mouse genome contains 14% more DNA than the human genome due to extensive duplications in the mouse lineage, as evidenced by syntenic regions.

Explanation: The study materials state that human chromosome 14 is largely identical to mouse chromosome 12, showing 'same genes in the same order', which defines synteny, and that approximately 180 breakage-and-rejoining events have occurred since their divergence over 100 million years ago. The materials also clarify that the mouse genome contains 14% less DNA than the human genome, not more, and that the loss of DNA is interspersed throughout the regions, not identical content. Finally, about 90% of the two genomes can be aligned in this way, not just 50%.

Question 3: The N-terminal tails of the core histones are located towards the outside of the nucleosome, are highly flexible, and can be covalently modified.

A. Ano

B. Ne

Explanation: The core histone proteins (H2A, H2B, H3, H4) have variable N-terminal tails that are directed towards the outside of the nucleosome. These N-terminal tails are highly flexible and are subject to numerous covalent modifications, such as methylation, phosphorylation, acetylation, or ubiquitylation.

Question 4: Which of the following statements accurately describe the process of chromatin spreading by reader-writer complexes?

A. A writer enzyme first modifies nucleosomes at a specific genomic site after being recruited by regulatory proteins.

B. The reader protein recognizes a different histone modification mark than the writer produces, leading to diverse spreading patterns.

C. The binding of the reader to the histone modification mark activates the writer, allowing the mark to spread to neighboring nucleosomes.

D. Barrier DNA sequences act to prevent the unlimited spreading of chromatin changes along a chromosome.

Explanation: The writer enzyme initially modifies nucleosomes at specific genomic sites following its recruitment, often by transcription regulatory proteins. For the spreading mechanism to work, the reader protein must recognize the *same* histone modification mark that the writer produces, and its binding to that mark activates the writer to spread the mark to adjacent nucleosomes. This spreading is not limitless, as barrier DNA sequences are present to halt the process.

Question 5: Is the median size of an exon in human genes greater than the median size of an intron?

A. Ano

B. Ne

Explanation: The study materials indicate that the median exon size is 131 nucleotide pairs, while the median intron size is 1747 nucleotide pairs. Therefore, the median exon size is not greater than the median intron size.