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DNA Replication, Protein Synthesis & Mendelian Heredity

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1. DNA Structure, Replication and Central Dogma

DNA consists of two antiparallel polynucleotide strands forming a double helix governed by precise base pairing and enzymatic replication machinery.

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Chemical structure of DNA double helix showing antiparallel strands, sugar-phosphate backbone, and base pairs

Wikimedia Commons / Madprime / CC BY-SA 3.0

Antiparallel Double Helix

Two complementary strands running in opposite directions (5'\to 3' and 3'\to 5')

Outer hydrophilic deoxyribose-phosphate backbone with interior nitrogenous bases

Complementary Base Pairing

Adenine with Thymine (2 hydrogen bonds); Guanine with Cytosine (3 hydrogen bonds)

In RNA transcript synthesis, Uracil (U) replaces Thymine (T)

Replication Enzymes

Helicase (unwinds) · Primase (RNA primer) · DNA Pol III (5'\to 3' synthesis) · Ligase (Okazaki sealing)

Semi-conservative mechanism: each daughter helix conserves one template strand

Worked example

1Analyze base composition using Chargaff's rules.
2A double-stranded DNA sample contains 28% Cytosine.
3By base pairing, % Guanine = 28%.
4Total G+C = 56%.
5Remaining A+T = 44%.
6Since % Adenine = % Thymine, % Adenine = 44% / 2 = 22%.
7Thus, Adenine = 22%, Thymine = 22%, Guanine = 28%, Cytosine = 28%.

1.Guanine pairs with Cytosine via three hydrogen bonds in double-stranded DNA.

2.DNA replication is conservative, preserving original intact double helices.

3.RNA Polymerase synthesizes RNA using Uracil instead of Thymine.

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