Nucleotides

Specification:• State that deoxyribonucleic acid (DNA) is a

polynucleotide, usually double stranded, made up of nucleotides containing the bases adenine (A), thymine (T), cytosine (C) and guanine (G);

• State that ribonucleic acid (RNA) is a polynucleotide, usually single stranded, made up of nucleotides containing the bases adenine (A), uracil (U), cytosine (C) and guanine (G);

Polynucleotides

• Nucelic acids come in two forms: DNA and RNA

• They hold all the information to make proteins which is what all living things are made up of

• Almost all of the DNA in a eukaryotic cell is found in the nucleus

• RNA is found in 3 different forms

Monomers

• The monomer of all nucleic acids (DNA and RNA) is called a nucleotide

• Each single nucleotide is made up of; one phosphate group, one sugar molecule, one organic nitrogenous base

• All three sub-units are joined by covalent bonds

Nucleotides

• There are five different nucleotides called A, T, C, G and U

• The phosphate group in all nucleotides is the same

• The sugar molecule is a 5 carbon sugar; either deoxyribose in DNA or Ribose in RNA

• The 5 possible bases are adenine; A, Thymine; T, Guanine; G, Cytosine; C, and Uracil; U

Joining nucleotides together• A condensation reaction occurs between the

phosphate group of one nucleotide and the sugar of another nucleotide

• Repeating the bonding gives a long chain of nucleotides

• This forms the ‘backbone’ of the molecule

Nucleotides to Nucleic Acids

• Chains of nucleic acids bond together to make nucleic acids

• Only nucleic acids carrying the same sugar (ribose or deoxyribose) can be binded together

• This makes nucleic acids either RNA or DNA

Too Much Nucleic Acid

• Uric acid is produced when excess purines (Adenine and Guanine) are broken down in the liver

• It is excreted in urine

• Some people have too much uric acid in the blood which is turned into crystals that are deposited at the joints making them swollen (this is called gout)

Task• Using Cambridge Biology

p124- 126, outline the structure of DNA- you must include the key words in bold in the paragraphs

• When finished, complete the DNA modelling task titled ‘Have Your DNA and Eat It’

• Then complete the questions on the printed slide- they will be peer assessed next lesson!

Questions

1. Outline how DNA replicates

2. Define the term anti-parallel

3. Write down the complimentary strand sequence for DNA and RNA- ATTAGGCTAT

4. If a DNA is 20% Thymine, what percentage of each of the other types would it contain?

5. What type of disease can result from DNA copying going wrong?

Questions1. Outline how DNA replicates semiconservatively- the double helix

is untwisted, the hydrogen bonds between the bases are broken apart and ‘unzip’ to expose the bases, free DNA nucleotides are hydrogen bonded onto the exposed bases according to the base pairing rule A-T G-C, covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone

2. Define the term anti-parallel prallel, but with chains running in opposite directions

3. Write down the complimentary strand sequence for DNA and RNA- ATTAGGCTAT DNA TAATCCGATA RNA UAAUCCGAUA

4. If a DNA is 20% Thymine, what percentage of each of the other types would it contain? 20% Adenine, 30% Cytosine, 30% Guanine

5. What type of disease can result from DNA copying going wrong? Cancer may occur if DNA is not copied properly

DNA- Information Storage

• DNA is a long chain polymer of nucleotide monomers

• The polymer is called a polynucleotide

• A DNA molecule forms when two polynucleotide molecules come together forming a ladder like structure

Hydrogen Bonding and Base Pairing

• The two DNA strands run parallel to each other• The term anti-parallel is used because the strands run in

opposite directions to one another• The chains are always the same distance apart because

bases pair up in a specific way.• When a purine appears on one side, a pyrimidine

appears on the other• Adenine always pairs with thymine, and guanine always

pairs with cytosine• As the strands come together, hydrogen bonds form

between the bases• The base pairing is described as complimentary

Double Helix

• In a complete DNA molecule, the anti-parallel chains twist, like twisting a rope ladder, to form the final structure known as a double helix

Making Copies

• DNA replication takes place during interphase of the cell cycle

• It is the process that creates identical sister chromatids

Making Copies

• In order to make a new copy of a DNA molecule:

• The double helix is untwisted• Hydrogen bonds between the

bases are broken apart to ‘unzip’ the DNA and expose the bases

• Free DNA nucleotides are hydrogen bonded onto the exposed bases according to the base pairing rules (A-T and C-G)

• Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone.

Making Copies

• This continues all the way along the molecule until two new DNA molecules (double helices) are formed, each is an exact replica of the original DNA molecule because of the base pairing rules.

• This process is known as semi-conservative replication.

• Each new DNA molecule consists of one conserved strand plus one newly built strand

How is the Structure of DNA related to its function?

• The sequence of bases acts as information storage in the form of codes to build proteins

• The molecules are long to store more information

• The base pairing means that complementary strands of information can be replicated

• The double helix gives the molecule stability

• Hydrogen bonds allow for easy unzipping for copying and reading information

Reading the Instructions

• RNA• Contains the sugar

ribose• Has uracil instead of

thymine• Usually single

stranded• Exists in 3 forms

Base Pairing of RNA

• Contains the purines adenine and guanin and the pyrimidines cytosine and uracil

• Base pairing of A-U and G-C occur

Three forms of RNA

• mRNA: messenger RNA is made as a strand that is complimentary to one strand of the DNA molecule (the template strand) it is therefore a copy of the other DNA strand as it contains the opposite bases plus Uracil

• rRNA: is ribosomal RNA and is found in ribosomes- it decodes mRNA into amino acids

• tRNA: transfer RNA carries amino acids to the ribosomes where they are bonded together to form polypeptides

Task: Now complete the Protein Synthesis sentence sort

Sentence Sort: What are the instructions for?

• The sequence of bases on DNA make up codes for particular proteins- they code for the sequence of amino acids

• The sequence coding for a particular protein (a gene) can be exposed by splitting the hydrogen bonds in the particular area of the DNA molecule

• RNA nucleotides from a complementary strand (mRNA) which is a copy of the DNA coding strand (or gene)

• The mRNA peels away from the DNA and leaves the nucleus through a nuclear pore

• The mRNA attaches to a ribosome• The tRNA molecules bring amino acids to the ribosome in the

correct order, according to the base sequence of the mRNA• The amino acids are joined by peptide bonds to give a protein with a

specific primary structure which can then give rise to the secondary and tertiary shape of the protein

Task

• Complete a windows movie explaining the steps involved in protein synthesis

• Success Criteria:• No more than one minute• Include scientific

information and diagrams• Explain simply the terms

transcription and translation

Questions

1. Explain why the MRNA strand produced in the nucleus is complementary to the template strand, and a copy of the coding strand

2. If a DNA template strand code reads ATTCGCGTTAAT, what would the complementary MRNA strand read?

3. Suggest why MRNA is less stable than DNA, and explain why this is a necessary feature of MRNA

4. Make a table to compare and contrast the structure of DNA with that of RNA

Questions1. Explain why the MRNA strand produced in the nucleus is complementary

to the template strand, and a copy of the coding strand complementary RNA nucleotides are lined up against each base on the template strand, producing a complementary strand. As base pairing rules apply, this lining up will be the same as it appears on the coding strand- apart from U in RNA replacing T in DNA

2. If a DNA template strand code reads ATTCGCGTTAAT, what would the complementary MRNA strand read? UAAGCGCAAUUA

3. Suggest why MRNA is less stable than DNA, and explain why this is a necessary feature of MRNA RNA is single stranded and so less stable, as nucleotide bases are exposed and not paired. It also contains uracil instead of thymine, which may contribute to the lower stability of the molecule. mRNA results in the production of proteins. If the cell is to control protein production, the disintegration of mRNA stops too much of a certain protein being made, and so allows for regulation of the protein levels in a cell

4. Make a table to compare and contrast the structure of DNA with that of RNA