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Nucleotides + DNA
Unit 2
Module 1…read more

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2.1.15 Nucleotides
· Nucleic Acids come in 2 forms ­ DNA + RNA. These hold the coded information to build that organism.
· The monomer of all nucleic acids is called a nucleotide.
· One nucleotide = 1 phosphate group + 1 sugar molecule + 1 organic nitrogenous base (covalently bonded).
· The phosphate group is always the same.
· The sugar molecule is a 5-carbon sugar ­ deoxyribose in DNA or ribose in RNA.
· There are 5 possible bases:
· To join 2 nucleotides together ­ a condensation reaction occurs between the
phosphate group of one and the sugar of another. Repeating this gives a long chain
of nucleotides ­ the backbone consists of a repeating sugar-phosphate chain. Organic bases project from the backbone.
· Chains of nucleotides bonded together are called nucleic acids.
· The 5 organic bases are grouped:
­ PURINES (bigger) = Adenine + Guanine
­ PYRIMIDINES (smaller) = Thymine + Uracil + Cytosine…read more

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2.1.16 DNA
· DNA is a long-chain polymer of nucleotide monomers ­ a polynucleotide.
· A DNA molecule forms when 2 polynucleotide strands come together ­ hydrogen bonds form between the bases (very
Hydrogen Bonding + Base Pairing
· 2 DNA strands run parallel to each other ­ antiparallel because the strands run in opposite directions to each other.
· The chains are always the same distance apart because the bases pair up in a specific way.
· Where a pyrimidine appears on one side, a purine appears on the other ­ GCAT (base-pairing rules).
· In a complete DNA molecule, the antiparallel chains twist to form the final structure ­ a double helix.
Making Copies
· DNA replication takes place in interphase of the cell cycle and is the process that creates identical sister chromatids.
· In order to make a new cop of a DNA molecule:
1. The double helix is untwisted.
2. Hydrogen bonds between the bases are broken apart to `unzip' the DNA ­ this exposes the bases.
3. Free DNA nucleotides are hydrogen-bonded onto the exposed bases according to base-pairing rules.
4. Covalent bonds are formed to seal the backbone.
5. This continues all the way along the molecule until 2 new DNA molecules are formed.
· Each new molecule will be an exact replica of the original DNA molecule.
· This process = semi-conservative replication because each new DNA molecule consists of 1 conserved strand + 1 newly built
· The DNA molecules are long so a large amount of information can be stored.
· Double helix structure gives the molecule stability.
· Hydrogen bonds allow easy unzipping for copying and reading information.…read more

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2.1.17 Reading the Instructions
· RNA is structurally different to DNA:
­ The sugar molecule = ribose.
­ The base Uracil is used instead of Thymine.
­ The polynucleotide chain is usually single-stranded.
­ 3 forms of RNA exist.
· GCAU base-pairing rules -> these mean that RNA molecules can be made that are complementary to DNA molecules.
Exposed DNA nucleotides can have free RNA nucleotides hydrogen-bonded to them and then the sugar-phosphate
backbone is sealed up to form a chain of RNA nucleotides ­ transcription.
3 forms of RNA
· MESSENGER RNA (mRNA) ­ made as a strand complementary to one strand of a DNA molecule so it is a copy of the coding
strand of the double helix.
· RIBOSOMAL RNA (rRNA) ­ found in ribosomes.
· TRANSFER RNA (tRNA) ­ carries amino acids to the ribosomes where they are bonded together to form polypeptides.
What are the Instructions for?
· The sequences of bases on DNA make up codes for particular protein molecules.
· RNA nucleotides form a complementary strand (mRNA) which is a copy of the DNA coding strand.
· The mRNA peels away from the DNA and leaves the nucleus ­ attaches to a ribosome.
· The tRNA molecules bring amino acids to the ribosome in the correct order, according to the base sequence on the mRNA.
· The amino acids are joined together to form a protein.…read more


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