Download 7.1 - DNA Structure

7.1 - DNA Structure
7.1.1 - Describe the structure of DNA, including the antiparallel strands, 3'-5' linkages and
hydrogen bonding between purines and pyrimidines
DNA has a uniform diameter along its entire length due to
complementary base pairing. The two polynucleotide chains
are antiparallel, with the polynucleotides formed around the
outside of the helix and the bases extending into the centre.
The chains held together by hydrogen bonding between the
bases on opposite nucleotides.
There is double hydrogen bonding between A and T. On the other hand, there is triple
hydrogen bonding between C and G. The purines are the nucleic bases with two rings:
adenine and guanine. On the other hand, the single-ringed bases are the pyrimidines,
thymine and cytosine.
7.1.2 - Outline the structure of nucleosomes
A nucleosome consists of DNA wrapped around eight histone
proteins and held together by another histone protein. The
DNA double helix has major and minor groves on the outer
diameter, exposing chemical groups that can form hydrogen
bonds. These groups are bonded to positively-charged
proteins called histones, forming two loops around them.
DNA is wound around and bonded to eight histones and secured by the H1 linker protein,
holding the DNA in place. This structure allows the long DNA molecules on the nucleus to be
condensed into a much smaller space. Together, the histones form ‘beads’. However, there
are also other proteins present in the chromosomes, including the enzymes for replication
and transcription.
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7.1.3 - State that nucleosomes help to supercoil chromosomes and help to regulate
transcription
During supercoiling, the DNA is condensed by a factor of x15000. The
histones are responsible for the packaging of DNA at the different levels.
The metaphase chromosome is an adaption for mitosis and meiosis. The
fibre must be less condensed for transcription to occur during interphase.
Condensing controls if the genes are transcribed or not.
7.1.4 - Distinguish between unique or single copy genes and highly repetitive sequences in
nuclear DNA
Unique or single copy genes form the gene coding region codes for
polypeptides, and make up about 3% of the human genome
The function of the non-coding region remains unclear. It is often made
up of highly repetitive sequences of bases, called satellite regions.
These are used in DNA fingerprint technologies.
7.1.5 - State that eukaryotic genes can contain exons and introns
Exon - a coding nucleotide sequence of the DNA of chromosomes
Intron - a non-coding nucleotide sequence of the DNA of chromosomes, present in
eukaryotic chromosomes
The highly repetitive sequences of introns, or satellite DNA, constitute 5-45% of the
genome, with between 5 and 300 base
pairs per repeat. These may be
duplicated as many as 105 times per
genome.
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