Functions and structures of DNA and nucleotide

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Learning objectives
11.4.1.8 establish a link between DNA structure and its

function
11.4.1.9 describe the chemical structure of the nucleotides and explain their connection and location in the DNA molecule

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success criteria
DNA:
1. Knows the structure of DNA.
2. Describes the functions of

DNA.
3. Establish a link between DNA structure to its function.
Nucleeotide:
1. Knows the chemical structure of a nucleotide.
2. Describes their connection.
3. Explain how the nucleotides are located in the DNA.

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Terminology
DNA/ nucleotide
Purines/pyremidines
Adenine/guanine/ cytosine/ thymine
Monomer/polymer
Phosphate group/pentose sugar/ deoxyribose/ nitrogenous-bases
H-bond/ covalent bond/

ester bond/ glycoside bond

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DNA
DNA stands for deoxyribonucleic acid. DNA is also polymer, made up

of many similar, smaller molecules joined into a long chain. The smaller molecules from which DNA molecules are made are nucleotides. DNA is therefore polynucleotides. They are often referred to simply as nucleic acids.

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DNA structure
The 3’ -5’ linkage
Antiparallel stands
The bases and hydrogen bonding
Nucleosomes

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The 3’ -5’ linkage
The carbons in the sugar are numbered from

1 to 5 in a clockwise direction starting after the oxygen at the apex.
The base is attached to carbons 1.
Carbon 2 has just a hydrogen attached instead of an OH group. (called - deoxyribose)
Carbon 3 is where te next nucleotide attaches in one direction.
Carbon 5 has a phosphate group attached to it.

This means that each nucleotide is linked to those on either side of it through carbons 3 and 5.

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Antiparallel stands
DNA molecules are made of two polynucleotide strands lying side

by side, running in opposite directions. The strands are said to be antiparallel.

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The bases and hydrogen bonding
The two strands are held together by

hydrogen bonds between the bases.
The bases can be purines or pyrimidines. From Figure, you will see that the two purine bases, adenine and guanine, are larger molecules than the two pyrimidines, cytosine and thymine.

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Hydrogen bonds
DNA molecules are made of two polynucleotide strands lying side

by side, running in opposite directions. The strands are said to be antiparallel. The two strands are held together by hydrogen bonds between the bases.

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Covalent bonds
Each nucleotide is linked to the next by covalent

bonds between the phosphates and sugars.

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Nucleosomes
A eukaryotic chromosome is composed of a double stand of DNA,

combined with proteins. This proteins called – histones, combine together in groups of either to form a bead – like structure.

It is held in place on the bead by a ninth histone.
The groupe of nine histone with the DNA is called a nucleosome.

The function of nucleosome: help to supercoil the chromosomes during mitosis, meiosis and help regulate transcription.

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Coding strand and non coding strand

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Nucleotide
Nucleotides are made up of three smaller components. These are:
a

nitrogen-containing base
a pentose sugar
a phosphate group.

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Nitrogen-bases
There are just five different nitrogen-containing bases found in DNA

and RNA.
In a DNA molecule, there are four: adenine, thymine, guanine and cytosine.
These bases are often referred to by their first letters: A, T, C, G and U.
This complementary base pairing is a very important feature of polynucleotides, as you will see later.

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Complementary base
Two polynucleotides, running in opposite directions, are held together

by hydrogen bonds between the bases.
A links with T by two hydrogen bonds;
C links with G by three hydrogen bonds.
This is complementary base pairing.

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Pentose sugar
The pentose (5-carbon) sugar can be either ribose (in

RNA) or deoxyribose (in DNA). As their names suggest, deoxyribose is almost the same as ribose, except that it has one fewer oxygen atoms in its molecule.

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Role of Phosphodiester linkage

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The components of nucleotides

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Type of bonds (types of links)

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Sugar – phosphate “backbone” and type of links.
In DNA it is

formed
of alternating sugars and phosphates linked together, with
the bases projecting sideways.
The covalent sugar–phosphate bonds (phosphodiester
bonds) link the 5-carbon of one sugar molecule and the
3-carbon of the next.

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