DNA STRUCTURE

TOPICS 3.3 & 7.1

Assessment Statements

3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate

3.3.2 State the names of the four bases in DNA3.3.3 Outline how DNA nucleotides are linked

together by covalent bonds into a single strand

3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds

3.3.5 Draw and label a simple diagram of the molecular structure of DNA

Nucleotides Subcomponents of DNA Composed of a

phosphate group, a sugar (deoxyribose), and a nitrogenous base

4 bases Adenine Thymine Guanine Cytosine

Purines – double ring structure Adenine and guanine

5’

4’

3’ 2’

1’

Pyrimidines – single ring structure

Thymine and cytosine

Nucleotides are linked covalently

DNA is a double helix and therefore composed of two strands

Each strand is shaped like a spiral staircase

Backbone of phosphate and deoxyribose that alternate and are held together by a phosphodiester bond or linkage

Double helix The two bases making

each rung are said to be complimentary to each other

A=T held together by 2 hydrogen bonds

C=G held together by 3 hydrogen bonds

animation

Assessment Statements

7.1.1 Describe the structure of DNA, including the antiparallel strands, 3’-5’ linkages and hydrogen bonding between purines and pyrimidines

7.1.2 Outline the structure of nucleosomes7.1.3 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

7.1.5 State that eukaryotic genes can contain exons and introns

How are the two strands of DNA held together?

The two sugar-phosphate chains are attached to each other by their nitrogenous bases held together by hydrogen bonds

The two chains run in opposite directions and are described as antiparallel

One strand has the 5’ carbon on the top and the 3’ carbon on the bottom; the other strand is the opposite

Nucleosomes Consists of two molecules of each of four

different histones DNA wraps twice around these eight protein

molecules DNA is negatively charged and histones are

positively charged A 5th type of histone leads to further wrapping

of DNA molecule When DNA is wrapped, it is inaccessible to

transcription enzymes Therefore the wrapping regulates the process

of transcription by allowing only certain areas of the DNA to be involved in protein synthesis

Types of DNA sequence

Highly repetitive (junk DNA) Composed of 5-300 bp per sequence May be as many as 100,000 replicates per

genome If clustered in discrete areas, it is referred to as

satellite DNA (ex. Centromere) Coding function? Are transposable First found by Barbara McClintock in 1950 Won Nobel Prize in Medicine in 1983

Protein-coding genes Single copy genes Provide base sequences essential to produce

proteins at the cell ribosomes Human Genome Project began in mid-1970s and

completed in 2001 Account for less than 2% of the chromosomes Not a fixed sequence; broken by non-coding

portions known as introns Coding fragments are exons

Structural DNA Highly coiled DNA that does not have a

coding function Occurs around centromere and near the

ends of chromosomes at the telomeres (responsible for cell aging?)

Inactive genes (pseudogenes) No function Caused by mutation?