Silent mutations will change a DNA sequence without affecting the phenotype of the organism. This can occur either in an intron, which will not be translated, or by replacing a single nucleotide with another nucleotide without changing the amino acid recruited by the codon. Silent mutations often result from the degenercy of codons.

Frameshift, missense, and nonsense mutations, however, change both an organism's genotype and phenotype by altering its DNA. A frameshift mutation results from the insertion or deletion of a nucleotide, causing a shift in the codon reading frame for every codon read after the mutation. Missense mutations replace one amino acid with another, and nonsense mutations result in a premature stop codon, terminating translation and resulting in a shortened protein.

Proofreading is a function of DNA polymerase III that helps prevent errors during replication. An immediate consequence of a cell that cannot proofread would be a higher rate of mutations during replication. The other options could potentially happen later in the cell's life, but they would only occur as indirect results of the new mutations.

Histones are composed of several proteins, and are used to compact DNA within the nucleus. When DNA is wrapped around a group of eight histones, the resulting structure is a nucleosome.

The histone protein H1 is affixed on top of the nucleosome beaded structure, so as to keep the DNA that has wrapped around the nucleosome in place. It is not found in the core of the nucleosome.

H2A, H2B, H3, and H4 are very similar in structure and form the core of the histones.

For correct mismatch repair all three of the choices are essential. A nuclease is required to remove the damaged DNA. DNA polymerase is required to synthesize new DNA. DNA ligase is essential for synthesizing a phosphodiester bond between the newly synthesized DNA and the original DNA.

Lipase is the general name for an enzyme that breaks down lipids. Ligase joins the Okazaki fragments on the lagging strand of the DNA during replication. DNA polymerase is the enzyme that catalyzes the polymerization of nucleotides in the 5' to 3' direction. Helicase separates the two strands of the double helix to facilitate formation of the replication bubble. Gyrase relieves strain on the DNA while it is being unwound by helicase. 

Proofreading is an important part of the DNA replication process to ensure that if mismatched base pairs are incorporated into the newly synthesized DNA strands, they get replaced with correct base paired nucleotides. Mismatched base pairs have the potential to cause disease. DNA polymerases have proofreading abilities. They are able to remove mismatched nucleotides from the end of a newly synthesized strand. Post-replication repair mechanisms also exist to prevent damage and error.

As the replication fork of DNA proceeds and continues to unwind the double helix, the DNA upstream of the fork gets over wound and knotted up which will eventually arrest replication as the fork will not be able to proceed any further. The enzyme topoisomerase corrects for this overwinding ahead of replication forks by swiveling and rejoining DNA strands

DNA and RNA share many characteristics. They are both composed of nucleotide monomers and are read in the 5'-to-3' direction. They also share the same complementary base pairs, except RNA uses uracil in place of thymine; both contain adenine.

RNA does not present in a double helix structure, and is typically single stranded.

Remember that mRNA is not a double helix like DNA; RNA is only one single strand of nucleotides. This means that we are unable to say that there are just as many cytosine bases as guanine bases, even though they would be able to form nucleotide pairs in DNA. For all we know, there could be zero cytosine bases! More information is needed before we can make a conclusion as to how many cytosine bases are in the RNA section.

The only correct statement here is the one regarding the types of sugar in the two molecules. RNA stands for "ribonucleic acid," which is a simple way to remember that it contains the sugar ribose. DNA, on the other hand, stands for "deoxyribonucleic acid." Its sugar is deoxyribose, which is identical to ribose except it is missing a hydroxyl (-OH) group on its second carbon. In total, RNA contains three hydroxyl groups, while DNA contains only two.

In RNA, uracil replaces thymine, not guanine. DNA is generally double-stranded and RNA is generally single-stranded (though both can exist in either form). Prokaryotes contain both DNA and RNA. Finally, DNA is transcribed to RNA in most biological organisms, but RNA can be reverse transcribed to DNA by the protein reverse transcriptase, which is found in some viruses.