DNA Structure and Function

DNA Structure

• The primary structure of DNA consists of a chain of nucleotides linked together by phosphodiester bonds.
• Each nucleotide is composed of a sugar, a phosphate group, and a nitrogenous base.
• The deoxyribose sugar in DNA has a hydrogen atom attached to the 2’ carbon atom.

DNA Secondary Structure

• The secondary structure refers to the helical arrangement of the DNA molecule, which is referred to as a double helix due to the two DNA strands joining and winding around each other.
• The bases are stacked on the inside of the helix, while the sugar-phosphate backbones are on the outside.
• The two strands run in opposite directions (antiparallel) with the 5’ end of one strand opposite the 3’ end of the other strand.
• Hydrogen bonds link the bases on opposite strands, forming A-T and C-G base pairs.

DNA Tertiary Structure

• The tertiary structure of DNA refers to the packing and arrangement of the double-stranded DNA within chromosomes.
• B-DNA is the most stable configuration of DNA, with the double helix forming a right-handed (clockwise) coil known as an alpha helix.

Nucleosomes

• Nucleosomes are fundamental units of chromatin, composed of a core particle of eight histone proteins (two each of H2A, H2B, H3, and H4).
• DNA is wrapped around the histone octamer, forming a nucleosome core particle.
• The histone H1 protein is associated with the linker DNA, forming a chromatosome.

Chromatin Structure

• Nucleosomes are linked together by linker DNA to form a helical structure termed the 30 nm fiber.
• The 30 nm fiber further folds into loops of 20,000 - 100,000 bp, about 300 nm long.
• These loops condense and fold to from a 250 nm fiber through further coiling.
• The 250 nm fiber forms the chromatid, about 700nm in width.

Centromeres

• Centromeres are constricted regions of chromosomes where spindle fibers attach during cell division.
• Centromeres are essential for proper chromosome movement during mitosis and meiosis.
• Centromeric DNA sequences are the binding sites for the kinetochore, to which spindle fibers attach.

Telomeres

• Telomeres are the natural ends of the chromosomes, consisting of repetitive nucleotide sequences (TTAGGG in humans).
• They provide a cap that stabilizes the chromosome, preventing it from unraveling or being degraded.
• They also play a role in the replication of the ends of the chromosome.

Chromatin vs. Chromatid

• Chromatin refers to the complex of DNA and protein that makes up chromosomes and is found throughout the cell cycle.
• Chromatid refers to one of two identical copies of a chromosome that are formed during DNA replication.
• Chromosomes are condensed chromatin fibers that are visible during cell division.

DNA Structure

• DNA is a macromolecule consisting of a polymer chain made up of repeating nucleotides.
• Nucleotides consist of a sugar (deoxyribose in DNA), a phosphate group, and a nitrogenous base.
• The phosphate groups link the sugars of adjacent nucleotides to create the backbone of the DNA molecule.
• The 5' end of a DNA strand has a free phosphate group at the 5' carbon of the sugar, while the 3' end has a free hydroxyl group at the 3' carbon.

Secondary Structure of DNA

• DNA exists as a double helix comprised of two polynucleotide strands running in opposite directions (antiparallel).
• The strands are held together by hydrogen bonds between complementary bases.
• Adenine (A) pairs with thymine (T) via two hydrogen bonds.
• Cytosine (C) pairs with guanine (G) via three hydrogen bonds.

Tertiary Structure of DNA in Prokaryotes

• Most bacterial genomes consist of a single circular DNA molecule.
• Bacterial DNA is not associated with histone proteins but is complexed to other proteins.
• This complex forms a distinct region within the bacterial cell called the nucleoid.

Tertiary Structure of DNA in Eukaryotes

• Eukaryotic DNA is tightly packaged with the help of proteins, forming chromatin.
• Chromatin exists in two forms: euchromatin (less condensed, actively transcribed) and heterochromatin (more condensed, inactive).
• The basic unit of chromatin is the nucleosome, consisting of a core particle of eight histone proteins (two each of H2A, H2B, H3, and H4) wrapped by 147 bp of DNA.
• The nucleosome core particle is bound by an H1 histone, forming the chromatosome, which encompasses about 167 bp of DNA.

Chromosome Structure

• Eukaryotic chromosomes are linear and are composed of a single, extremely long DNA molecule.
• The centromere is a constricted region where spindle fibers attach, essential for proper chromosome movement during cell division.
• Telomeres are protective caps at the ends of chromosomes, preventing degradation and aiding in replication.

DNA Packaging Summary

• Nucleosomes fold to form a 30 nm fiber, which further folds into loops.
• These loops are further condensed and folded to produce a fiber with a diameter of 250 nm.
• Finally, tight helical coiling of the 250 nm fiber gives rise to a chromosome.

Key Terminology

• Chromatid: One half of a duplicated chromosome.
• Chromatin: Complex of DNA and protein found in eukaryotic cells.
• Chromatosome: Nucleosome bound to an H1 histone.
• Nucleosome: Core particle of eight histone proteins wrapped with DNA.
• Nucleoid: Region in bacterial cells containing DNA.
• Telomere shortening: Gradual reduction in telomere length associated with aging.

This quiz explores the various structures of DNA, including its primary, secondary, and tertiary formations. Understand the composition of nucleotides, the double helix configuration, and how DNA is organized within chromosomes. Perfect for biology students looking to test their knowledge of molecular genetics.