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.

Many genes in eukaryotes contain introns. These are non-coding sequences which are transcribed but not translated. The sequences that are not removed, that make up mature mRNA, are called exons. Prokaryotes do not usually have introns.

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