Microbiology Lecture 2: DNA Structure

Structure of DNA

• A DNA molecule is composed of repeating units called nucleotides, each consisting of a nucleobase, deoxyribose, and a phosphate group.
• The sugar-phosphate backbone of DNA has a nitrogenous base attached to each sugar, and these bases always pair in a specific way: adenine with thymine and cytosine with guanine.

Double Helix Structure

• The two strands of DNA are antiparallel, meaning one strand's sugar-phosphate backbone is upside down relative to the other strand.
• The two strands are held together by hydrogen bonds between their nitrogenous bases, making them complementary.

DNA Replication

• DNA replication involves the conversion of one parental double-stranded DNA molecule into two identical daughter molecules.
• The complementary structure of nitrogenous base sequences allows one strand to act as a template for the production of the other strand.

Process of DNA Replication

• The double helix of parental DNA separates, and hydrogen bonds form between new complementary nucleotides and each strand of the parental template.
• Enzymes catalyze the formation of sugar-phosphate bonds between sequential nucleotides on each resulting daughter strand.

Enzymes Involved in DNA Replication

• Topoisomerase or gyrase relaxes supercoiling ahead of the replication fork.
• Helicase unwinds the double-stranded DNA, and enzymes match free nucleotides to the exposed bases of the single-stranded parental DNA.
• DNA polymerase synthesizes DNA, proofreads, and repairs DNA.
• Other enzymes involved in DNA replication, expression, and repair include DNA ligase, endonucleases, exonucleases, methylase, photolyase, primase, ribozyme, RNA polymerase, snRNP, and transposase.

Genes and RNA Transcription

• Genes are composed of exons (expressed regions) and introns (intervening regions that do not encode protein).
• RNA polymerase synthesizes an RNA transcript containing copies of introns, and snRNPs remove the introns and splice the exons together.