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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