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7.1 dna structure
 

7.1 dna structure

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    7.1 dna structure 7.1 dna structure Presentation Transcript

    • 7.1 DNA Structure Topic 7 Nucleic Acids & Proteins
    • DNA Structure  7.1.1 Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines.  (Major and minor grooves, direction of the “twist”, alternative B and Z forms, and details of the dimensions are not required).  7.1.2 Outline the structure of nucleosomes. (Limit this to the fact that a nucleosome consists of DNA wrapped around eight histone proteins and held together by another histone protein).
    • DNA Structure  7.1.3 State that nucleosomes help to supercoil chromosomes and help to regulate transcription.  7.1.4 Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.  Highly repetitive sequences (satellite DNA) constitutes 5– 45% of the genome. The sequences are typically between 5 and 300 base pairs per repeat, and may be duplicated as many as 105 times per genome.
    • DNA Structure  TOK: Highly repetitive sequences were once classified as “junk DNA”, showing a degree of confidence that it had no role. This addresses the question: To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?  7.1.5 State that eukaryotic genes can contain exons and introns.
    • DNA Structure - Revision  Some Revision:  DNA is a double helix structure  Made up of nucleotides. A nucleotide consists of a phosphate group, a deoxyribose sugar and a nitrogenous base.  There are 4 nitrogenous bases which show complementary base pairing:  Adenine – Thymine  Cytosine – Guanine  The double helix structure of DNA is like a ladder twisted into a corkscrew shape around its longitudinal axis.  The strands of the ladder run in opposite directions.
    • Ref: Senior Biology 1, Biozone
    • DNA Structure  Some Key points:  Cytosine and Thymine are Pyrimidines (they all have a Y)  Adenine and Guanine are Purines.  There are Hydrogen bonds between the bases  A T has two  G C has three (Gee Cee Three)  The sugar-phosphate backbones form anti-parallel strands.  The sugars are linked by phosphate groups attached to carbons 3 and 5 of the sugar (for both ribose and deoxyribose sugars).  These are called 3’ – 5’ linkages.
    • Ref Biology for the IB Diploma, Allott
    • Nucleosomes  Analysis of chromosomes has shown that they are made up of DNA and protein.  The total length of DNA in a human cell is about 2.2 m.  To pack this into a nucleus means that the total length must be shortened to about 0.2mm.  This means that the length is reduced by a factor of about 8000, which needs good organisation.  DNA is wrapped around special proteins called histone proteins forming a structure called a Nucleosome.
    • Nucleosomes  A nucleosome consists of DNA wrapped around 8 histone proteins and a 9th histone protein to tie it off.
    • Nucleosome Organisation  For DNA to reduce its length, coiling and super coiling of the chromatin must occur - nucleosomes are an essential part of this organisation.  Nucleosomes help to regulate transcription. Ref: Advanced Biology, Roberts, Reiss & Monger
    • DNA in Eukaryotes & Prokaryotes  In eukaryotes DNA is associated with proteins to form nucleosomes, whereas in prokaryotes the DNA is naked.  Much of the DNA in eukaryotes consists of repetitive base sequences. These sequences are not genes.. They can however, be useful for DNA profiling, but their role is uncertain. Prokaryotes do not usually have repetitive sequences.  7.1.4 Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.  Highly repetitive sequences (satellite DNA) constitutes 5– 45% of the genome. The sequences are typically between 5 and 300 base pairs per repeat, and may be duplicated as many as 105 times per genome.
    • Nucleosome Organisation Ref: Advanced Biology, Kent
    • DNA in Eukaryotes & Prokaryotes  In eukaryotes DNA is associated with proteins to form nucleosomes, whereas in prokaryotes the DNA is naked.  Much of the DNA in eukaryotes consists of highly repetitive base sequences. These sequences are not genes.. They can however, be useful for DNA profiling, but their role is uncertain. Prokaryotes do not usually have repetitive sequences.
    • DNA in Eukaryotes  Many genes in eukaryotes contain introns. These are non-coding sequences which are transcribed but not translated. The sequences that are not removed are called exons.
    • DNA Structure  7.1.1 Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines.  (Major and minor grooves, direction of the “twist”, alternative B and Z forms, and details of the dimensions are not required).  7.1.2 Outline the structure of nucleosomes. (Limit this to the fact that a nucleosome consists of DNA wrapped around eight histone proteins and held together by another histone protein).
    • DNA Structure  7.1.3 State that nucleosomes help to supercoil chromosomes and help to regulate transcription.  7.1.4 Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.  Highly repetitive sequences (satellite DNA) constitutes 5– 45% of the genome. The sequences are typically between 5 and 300 base pairs per repeat, and may be duplicated as many as 105 times per genome.
    • DNA Structure  TOK: Highly repetitive sequences were once classified as “junk DNA”, showing a degree of confidence that it had no role. This addresses the question: To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?  7.1.5 State that eukaryotic genes can contain exons and introns.
    • Thank you to Mr Taylor at i-Biology for two of these slides