DNA Structure and Replication

DNA Structure and Replication

• DNA is a double helix with two strands held together by hydrogen bonds between complementary bases (Watson-Crick base pairs).
• The strands run antiparallel (5' to 3' and 3' to 5').
• Nucleotides are composed of a sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).
• Deoxyribose lacks an oxygen atom at the 2' position, hence the name deoxyribonucleic acid (DNA).
• Ribose has an oxygen atom at the 2' position.
• Nitrogenous bases are either purines (adenine, guanine) or pyrimidines (cytosine, thymine).
• Purines have a double ring structure, while pyrimidines have a single ring structure.
• Nucleotides can have up to three phosphate groups attached.
• Nucleosides consist of a base and a sugar.
• Nucleotides consist of a nucleoside and one, two, or three phosphate groups.
• DNA is identified by the lack of a 2' OH group, present in the ribose sugar , but not in the deoxyribose.

DNA Replication

• DNA replicates using a template strand, creating a new complementary strand.
• DNA polymerase synthesizes DNA in the 5' to 3' direction, adding free nucleotides that are complementary to the template strand. This always happens at the 3′ end so the new strand grows in the 5′ → 3′ direction.
• DNA primase synthesizes RNA primers, extending in the 3' direction.
• Replication starts at specific origins of replication.
• Helicase unwinds the DNA double helix, creating replication forks.
• Topoisomerase relieves the torsional strain caused by unwinding.
• Replication proceeds bi-directionally from each origin.
• The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments (Okazaki fragments). DNA ligase joins these.
• DNA polymerase requires deoxynucleoside triphosphates.
• DNA synthesis only happens at the 3' end of the growing strand.

DNA Replication Enzymes

• DNA polymerase: synthesizes new DNA strands.
• DNA primase: synthesizes RNA primers.
• Helicase: unwinds the DNA double helix.
• Topoisomerase: relieves torsional strain during unwinding.
• DNA ligase: joins Okazaki fragments.

Additional Concepts (Based on Questions)

• Specialised cell types: Specialized cells result from differences in transcription factors, turning on/off different genes.
• Transcription start sites: These sites contain a specific nucleotide sequence (e.g., TATAAT) for the attachment of transcription factors, controlling gene expression.

Explore the intricate structure of DNA and the process of its replication. This quiz covers essential concepts like the double helix formation, nucleotide components, and the antiparallel nature of DNA strands. Understand the differences between deoxyribose and ribose, as well as purines and pyrimidines.