Nucleic Acids 2.3

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Nucleic Acids

DNA (deoxyribonucleic acid)
Nucleotides are molecules that occur in nearly all biological processes. Nucleotides consist of a phosphate, a pentose sugar and a nitrogenous base. These molecules are monomers which become polymers by attaching to each other with a covalent bond called an ester bond. They attach by a condensation reaction.

-Are the monomers of DNA and RNA. In DNA the sugar is deoxyribose, in RNA the sugar is ribose.
-When a nucleotide gets more than on phosphate it becomes phosphorylated. ATP is an example of this, adenine triphosphate is an energy rich end product of most energy releasing biochemical reactions.
-Nucleotides help regulate metabolic pathways (ATP)
-Nucleotides can also be components of coenzymes involved in respiration and photosynthesis

Deoxyribonucleic acid (DNA) is found in the nucleus of all eukaryotic cells and within the cytoplasm of prokaryotic cells. DNA is a macromolecule that helps make up the structure of all living organisms. Other macromolecules are carbohydrates, lipids and proteins.

Structure of DNA
DNA is a polymer of nucleotides with a deoxyribose pentose sugar. DNA consists of two strands of polynucleotide strands. Each DNA molecule has one of four nitrogenous bases; adenine, guanine, cytosine and thymine. The bond between the phosphate group and the sugar in the nucleotide is called a phosphodiester bond. DNA molecules are long so that they can carry a lot of encoded genetic information.

Purines and Pyrimidines
There are two categories that split the four organic bases in the nucleotide. Adenine and Guanine are purines and have two rings to their structure, Cytosine and Thymine are pyrimidines and only have one ring to their structure.

Hydrogen Bonds
The antiparallel strands of DNA are joined to each other by hydrogen bonds. The hydrogen bonds are between the nitrogenous bases. Adenine pairs with thymine with two hydrogen bonds, guanine pairs with cytosine with three hydrogen bonds. Purines always pair with pyrimidines for stability. Hydrogen bonds are weak yet effective giving the DNA the ability to unzip for replication.

Anti-parallel sugar phosphate backbones
The back bone of DNA is made from a sugar phosphate. The opposite directions of the back bones refer to the direction that the third and fifth carbon molecules are facing. The 5 refers to the pentose sugar being attached to the fifth carbon and the same applies for the third.

How DNA is organised in cells
Eukaryotic cells:
-The DNA is in the nucleus
-The DNA is wrapped around histone proteins similar to cotton or wool around a bobbin into chromosomes. Each chromosome is one molecule of DNA.
-There is a DNA loop without histone proteins inside the mitochondria and chloroplasts

Prokaryotic cells:
-DNA is in a loop in the cytoplasm
-The DNA is naked
-Viruses can contain DNA in naked form

How DNA replicates
DNA is self-replicating
All DNA carries coded instructions of how to make and maintain an organism. Every time a cell divides the DNA is copied so that the daughter cell has the


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