BIO206H5 Chapter Notes - Chapter 5: Dna Replication, Noncoding Dna, Y Chromosome

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18 Feb 2016
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Chapter 5 – DNA and Chromosomes
Despite its chemical simplicity, the structure and chemical properties of DNA make it ideally suited for
carrying genetic information
THE STRUCTURE OF DNA
Well before biologists understood the structure of DNA, they had recognized that inherited traits and the
genes that determine them were associated with chromosomes
DNA carries the hereditary information of the cell and that the protein components of chromosomes
function largely to package and control the enormously long DNA molecules
The helical structure of DNA immediately suggested how DNA could encode the instructions necessary
for life, and how these instructions could be copied and passed along when cells divide
A DNA Molecule Consists of Two Complementary Chains of Nucleotides
A molecule of deoxyribonucleic acid (DNA) consists of two long polynucleotide chains each chain is
composed of four types of nucleotide subunits, and the two strands are held together by hydrogen bonds
between the base portions of the nucleotides
Nucleotides are composed of a nitrogen-containing base and a five-carbon sugar, to which is attached
one or more phosphate groups
oIn DNA, the sugar is deoxyribose and the bases can be either adenine (A), cytosine (C), thymine
(T), or guanine (G)
oThe nucleotides are covalently linked together in a chain through the sugars and phosphates
The way in which the nucleotide subunits are linked together gives a DNA strand a chemical polarity
The two ends of the strand can be easily distinguished – the 5’ phosphate and the 3’ hydroxyl ends
The two polynucleotide chains in the DNA double helix are held together by hydrogen bonding between
the bases on the different strands (A always pairs with T, C always pairs with G)
In each case, a two-ring purine is paired with a single-ring pyrimidine
Each purine-pyrimidine pair is called a base pair, and this complementary base pairing enables the base
pairs to be packed in the energetically most favourable arrangement in the interior of the double helix
The members of each base pair can fit together within the double helix because the two strands of the
helix run antiparallel to each other
A consequence of the base-pairing requirements is that each stand of a DNA double helix contains a
sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand
This complementarity is crucial when it comes to both copying and repairing DNA
The Structure of DNA Provides a Mechanism for Heredity
Information is encoded in the order, or sequence, of the nucleotides along each DNA strand
Organisms differ from one another because their respective DNA molecules have different nucleotide
sequences that carry different biological messages
The linear sequence of nucleotides in a gene must be able to spell out the linear sequence of amino acids
in a protein
The genetic code is the exact correspondence between the 4-letter nucleotide alphabet of DNA and the
20-letter amino acid alphabet of proteins
Gene expression is the process by which the nucleotide sequence of a gene is transcribed into the
nucleotide sequence of an RNA molecule, which in most cases is then translated into the amino acid
sequence of a protein
THE STRUCTURE OF EUKARYOTIC CHROMOSOMES
Each human cell contains about 2 meters of DNA; yet the cell nucleus is only 5-8µm in diameter
In eukaryotic cells, very ling double-stranded DNA molecules are packaged into chromosomes
oThese DNA molecules not only fit readily inside the nucleus, but, after they are replicated, they
can be easily apportioned between the two daughter cells at each cell division
The complex task of packaging DNA is accomplished by specialized proteins that bind to and fold the
DNA, generating a series of coils and loops that provide increasingly higher levels of organization and
prevent the DNA from becoming a tangled, unmanageable mess
The DNA is compacted in a way that allows it to remain accessible to all of the enzymes and other
proteins that replicate it, repair it, and control the expression of its genes
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