2.6 The structure of DNA
Lesson one: The structure of DNA
Objective: Understand the nature of nucleotides, and how they can be assembled to form DNA and also RNA
- Nucleotide: a building block for polynucleotides (eg. DNA and RNA)
- Deoxyribose: the pentose sugar found in DNA (carbon 2 is missing an oxygen atom)
- Ribose: the pentose sugar found in RNA (carbon 2 has a hydroxide group including an oxygen atom)
- Nitrogenous base: part of a nucleotide, may be a purine (2 ring base such as Adenine and Guanine, or a pyrimidine (single ring base such as cytosine and thymine)
introductory video (note some details are not required at IB level eg. B-form DNA)
DNA is made from the sub-units nucleotides. One nucleotide consists of a ribose sugar (pentose), a phosphate group, and a nitrogenous base.
To join the nucleotides together into a chain, strong covalent bonds are formed between the phosphate and ribose of adjacent nucleotides.
Skill: Draw a nucleotide, showing the detail of the carbon atoms in the ribose, and the phosphate group.
Skill: Identify the 3′ and the 5′ end of a nucleotide, based on what you know of the numbering system for carbon atoms in a sugar. Be able to identify the 3′ and 5′ end of a chain of nucleotides.
Assembling a DNA molecule:
There are four nitrogenous bases found in DNA:
- Adenine (A)
- Guanine (G)
- Thymine (T)
- Cytosine (C)
These bases are paired in DNA structure, according to the following base pair rules:
Weak hydrogen bonds form between the nitrogenous bases, assembling an anti-parallel double helix.
Strong covalent bonds join the 3′ end of a deoxyribose sugar to the phosphate of another nucleotide. These bonds are called phosphodiester linkage)
The complete is often shown as a 2-D flat formation merely to simplify the structure, although in reality this shape is twisted into a double helix.
The above molecule is twisted to form a double helix:
A gene may contain as many as 27,000 base pairs. Typically only a section of about 1,000 base pairs is copied to make single stranded RNA.
Ribose Nucleic acid is a single stranded copy of the sense strand of DNA (one of the two strands, that codes for a protein). The rest of the DNA is important for regulation of gene expression eg. promoter genes (which activate genes).