Structure of nucleic acids
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Describe the structure of a nucleotide, including its three components.
A nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. These components are covalently bonded together.
Distinguish between purines and pyrimidines, naming the bases that belong to each group.
Purines (adenine and guanine) have a double-ring structure, while pyrimidines (cytosine, thymine in DNA, and uracil in RNA) have a single-ring structure. This structural difference is crucial for proper base pairing in DNA and RNA.
Explain the importance of complementary base pairing in the structure of DNA, and how this relates to the antiparallel nature of the strands.
Complementary base pairing (A with T, and C with G) ensures accurate DNA replication and transcription. The antiparallel nature (5' to 3' strand paired with a 3' to 5' strand) allows for optimal hydrogen bond formation between the base pairs, stabilizing the double helix.
Compare the number of hydrogen bonds between C-G and A-T base pairs in DNA.
A-T base pairs are connected by two hydrogen bonds, while C-G base pairs are connected by three hydrogen bonds. The greater number of hydrogen bonds in C-G pairs contributes to the greater stability of DNA regions with high C-G content.
Describe the role of DNA polymerase in semi-conservative DNA replication.
DNA polymerase is the enzyme responsible for synthesizing new DNA strands by adding nucleotides complementary to the template strand. It can only add nucleotides to the 3' end of an existing strand, hence the 5' to 3' directionality.
Explain the difference between the leading and lagging strands during DNA replication.
The leading strand is synthesized continuously in the 5' to 3' direction towards the replication fork. The lagging strand is synthesized discontinuously in short fragments (Okazaki fragments) because DNA polymerase can only add nucleotides in the 5' to 3' direction, away from the replication fork.
What is the function of DNA ligase in DNA replication?
DNA ligase is an enzyme that joins the Okazaki fragments on the lagging strand during DNA replication, forming a continuous DNA strand. It catalyzes the formation of phosphodiester bonds between the fragments.
Describe the structure of an mRNA molecule.
mRNA is a single-stranded RNA molecule that carries genetic information from DNA to the ribosomes for protein synthesis. It contains a sequence of codons, each of which specifies a particular amino acid.
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