DNA Structure and Function

Cellular Molecular biology

What are the three parts of a nucleotide, and how are nucleotide monomers added to form a nucleic acid strand?

A nucleotide has (1) a phosphate group, (2) a five-carbon sugar (deoxyribose or ribose), and (3) a nitrogenous base. Nucleotides link by phosphodiester bonds between the 3’ OH of one sugar and the 5’ phosphate of the next, forming a sugar-phosphate backbone. The strand has a 5’ end (free phosphate) and a 3’ end (free OH).

How do DNA and RNA structures differ?

DNA has deoxyribose sugar, is usually double-stranded, and uses thymine (T). RNA has ribose sugar, is usually single-stranded, and uses uracil (U) instead of thymine.

What is the difference between purines and pyrimidines, and what are the five bases in nucleic acids?

Purines (double-ring): adenine (A), guanine (G). Pyrimidines (single-ring): cytosine (C), thymine (T), uracil (U).

Describe the structure of the DNA double helix.

DNA consists of two antiparallel strands forming a right-handed helix. Bases pair inside (A-T, G-C), sugar-phosphate backbones face outside. One strand runs 5’→3’, the other 3’→5’.

How does hydrogen bonding drive complementary base pairing?

A pairs with T (2 hydrogen bonds), and G pairs with C (3 hydrogen bonds). This specific pairing ensures accurate replication and transcription.

How do hydrogen bonding and van der Waals forces stabilize the double helix?

Hydrogen bonds hold complementary bases together. Stacking interactions (van der Waals forces) between adjacent bases stabilize the helix further.

What are the three main functions of double-stranded DNA?

(1) Store genetic information, (2) allow replication for inheritance, and (3) provide a template for RNA and protein synthesis.

What are the two main forces stabilizing DNA structure?

Hydrogen bonding between base pairs and base-stacking interactions (van der Waals forces).

How is the prokaryotic genome structured and packaged?

Prokaryotes usually have a single circular chromosome, compacted by supercoiling and proteins into a nucleoid (not membrane-bound).

Describe eukaryotic genome structure and the nucleosome/30 nm fiber.

Eukaryotic DNA is linear, packaged with histone proteins. A nucleosome = DNA wrapped around histone octamer. Nucleosomes fold into a 30 nm fiber for higher-order compaction.

How does eukaryotic DNA packaging differ between interphase and metaphase

Interphase: DNA loosely packed (chromatin) for transcription and replication. Metaphase: DNA highly condensed into visible chromosomes for segregation.

What is the anatomy of a eukaryotic chromosome? Include telomeres and centromeres.

Chromosomes have a centromere (links sister chromatids, attachment for spindle fibers) and telomeres (repetitive sequences at ends protecting against degradation).

What is the difference between euchromatin and heterochromatin?

Euchromatin: loosely packed, transcriptionally active. Heterochromatin: tightly packed, transcriptionally inactive.

How do interphase chromosomes contain both condensed and extended chromatin?

Chromosomes have regions of heterochromatin (condensed, inactive) and euchromatin (extended, active) within the same nucleus, allowing regulation of gene expression.