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In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid (DNA).
Hereditary information is encoded in DNA and reproduced in all cells of the body.
The DNA program directs the development of biochemical, anatomical, physiological, and, to some extent, behavioral traits.

Early in the 20th century, identifying the molecules of inheritance was a significant challenge for biologists.
T. H. Morgan’s group demonstrated that genes are located on chromosomes, making the two components of chromosomes—DNA and protein—candidates for the genetic material.

DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group.
The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C).
In 1950, Erwin Chargaff reported that DNA composition varies between species, contributing to its credibility as the genetic material.

Two main points known as Chargaff’s rules: - The base composition of DNA varies between species. - In any species, the number of adenine (A) and thymine (T) bases is equal, and the number of guanine (G) and cytosine (C) bases is equal.
The basis for these rules was understood following the discovery of the double helix.

After DNA was accepted as the genetic material, a major challenge emerged: determining how its structure accounts for its role in heredity.
Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study molecular structure, producing a significant picture of the DNA molecule.

Franklin’s images of DNA enabled Watson to deduce the molecule's helical structure, its width, and the spacing of the nitrogenous bases.
The pattern suggested that the DNA molecule was composed of two strands forming a double helix.

Franklin never collaborated with Watson and Crick; her data was shown to them without her knowledge by Wilkins.
It was noted that Watson and Crick's model conformed to the evidence available from the X-rays and chemistry of DNA.

Watson built a model depicting antiparallel backbones, which means their subunits run in opposite directions.
Initially, Watson and Crick assumed bases paired similarly (A with A), but this led to inconsistent widths.
Realizing that pairing a purine (A or G) with a pyrimidine (C or T) resulted in a consistent width led to correct base pairing rules: - Adenine (A) pairs exclusively with thymine (T). - Guanine (G) pairs exclusively with cytosine (C).
This specificity explained Chargaff’s rules: the amount of A = T and the amount of G = C in any organism.

The interrelationship of structure and function is evident in the double helix structure identified by Watson and Crick.
The specific base pairing suggested a potential copying mechanism for genetic material.

Each DNA strand is complementary, serving as a template for building a new strand during replication.
In replication, the parent molecule unwinds, and daughter strands are constructed based on base-pairing rules.

Watson and Crick proposed a semiconservative model, predicting that each daughter molecule consists of one old strand (conserved from the parent) and one newly synthesized strand.
Competing models: - Conservative model: two parent strands rejoin. - Dispersive model: each strand is a mix of old and new.

Replication starts at specific sites known as origins of replication.
Eukaryotic chromosomes may have numerous origins, allowing replication to proceed bidirectionally until the entire molecule is copied.

At each replication bubble, a replication fork is formed, indicating new DNA strands' elongation.
Key enzymes include: - Helicase: untwists the double helix at replication forks. - Single-strand binding proteins: stabilize single-stranded DNA. - Topoisomerase: alleviates twisting strain by breaking and rejoining DNA strands.

DNA polymerases synthesize new DNA at the replication fork, requiring a primer, which is a short RNA sequence created by the enzyme primase.
The primer is short (5-10 nucleotides long), with its 3′ end as the starting point for the new DNA strand.
DNA polymerases can only extend strands in the 5′ to 3′ direction, which influences the overall replication process.

DNA polymerase synthesizes a leading strand continuously toward the replication fork.
The lagging strand is synthesized in segments known as Okazaki fragments, which are joined later by DNA ligase.

DNA polymerases proofread the newly synthesized DNA, correcting incorrect nucleotides.
Mismatch repair processes rectify base pairing errors.
DNA can be damaged by chemical agents or undergo spontaneous changes; nucleotide excision repair removes damaged regions.

Despite effective proofreading and repair, errors can occur, leading to mutations, which serve as genetic variation sources for evolution.

DNA polymerase difficulties pose issues for linear eukaryotic DNA, leading to shorter DNA strands with each replication.
Prokaryotic DNA, being circular, does not face this problem.

Eukaryotic chromosomes feature specialized nucleotide sequences at their ends known as telomeres.
Telomeres help mitigate the loss of essential genes during replication, though they do not prevent shortening.
Telomere shortening is associated with aging, while the enzyme telomerase can lengthen telomeres in germ cells.

A chromosome is defined as a DNA molecule associated with proteins. Eukaryotic chromosomes have linear DNA associated with a substantial amount of protein, while bacterial chromosomes feature circular DNA with minor protein interaction.
In eukaryotes, DNA combines with proteins to form chromatin, characterized by a highly organized packing system within the nucleus.

Nucleosomes, the fundamental units of DNA packaging, consist of DNA wrapped around histone proteins.
Chemical modifications to histones can influence chromatin condensation and affect gene expression.

Chromatin undergoes regular packing changes throughout the cell cycle, transitioning from a less condensed state during interphase to a densely packed formation in preparation for mitosis.