Module 2, Section 3, Nucleotides and Nucleic Acids

They are made up of elements C,H,O,N and P

Monomers of DNA (used to store genetic information) and RNA (used to make proteins)

ADP and ATP are special types of nucleotides used to store and transport energy.

It is called deoxyribose.

DNA always contains the same sugar and phosphate group but the base can vary.  

There are four possible bases:

• Adenine 
• Guanine
• Cytosine
• Thymine

A and G are purine (they contain 2 rings) while C and T are pyrimidine (they only contain 1 ring)

A molecule of DNA contains 2 polynucleotide chaine - each chain is made up of lots of nulceotides joined together. 

It is called Ribose.

Also always has the same sugar and phosphate group but the nitrogenous base changes.

In RNA, it also contains Adenine, Guanine and Cytosine like DNA, but instead of Thymine it contains the base Uracil. 

RNA is made up of only one polynucleotide chain. 

ADP

• contains the base adenine
• and sugar ribose
• and 2 phosphate sugars

ATP

• conains the base adenine
• and sugar ribose
• and 3 phosphate sugars

ATP provides energy. It is synthesised from ADP and an inorganic phosphate using the energy from an energy releasing reaction.

The ADP is phosphorylated to form ATP and a phosphate bond is formed.

Energy is stored in the phosphate bond which when released is used by the cell. 

Nucleotides join together between the phosphate group of one to the sugar of another forming a phosphodiester bond.

The chain of sugars and phosphates is known as the Sugar-Phosphate Backbone.

Forming a Double Helix

Two polynucleotide strands join together by hydrogen bonds between the bases. The bases always join by complementary base pairing:

• A to T (2 hydrogen bonds between these bases)
• C to G (3 hydrogen bonds between these bases)

A purine always pairs with a pyrimidine.

2 antiparallel polynucleotide strands twist to form the DNA double helix

DNA has to copy itself before cell division so each new cell has the full amount of DNA:

1) DNA gyrase unwinds the strands. DNA helicase then unzips the strand by breaking hydrogen bonds forming two strands.

2) Each original strand acts as a template for a new strand. Free-floating DNA bases join to the exposed bases by complimentary base pairing.

3) Nucleotides of new strands join together by enzyme DNA polymerase. They form the backbone and the strands twist to reform the double helix.

4) Each new DNA strand contains one new and one original strand.

• This type of copying is called semi-conservative because it contains one original strand.
• Every so often a random, spontaneous mutation occurs. A mutation is any change to the DNA base sequence. Mutations don’t always have an effect, but they can alter the sequence of amino acids in a protein. This can cause an abnormal protein which may or may not have an effect. 

A gene is a sequence of DNA nucleotides that codes for a specific protein.

Different proteins have a different number and order of nucleotide bases/amino acids.

Each amino acid is coded for by three bases in a gene. (triplet)

DNA has to be copied into RNA for protein synthesis. 

Messenger RNA (mRNA)

• Made in the nucleus.
• Three adjacent bases are called a codon.
• Carries genetic code from the DNA in the nucleus to the cytoplasm, where it is used to make a protein during translation. 

Transfer RNA (tRNA)

• Found in the cytoplasm
• Amino acid binding site on one end and a sequence of three bases called an anticodon on the other end.
• Carries the amino acid that is used to make proteins to the ribosome during translation.

Ribosomal RNA (rRNA)

• Forms the two subunits in a ribosome.
• The ribosome moves along the mRNA strand during protein synthesis. The rRNA in the ribosome helps to catalyse the formation of peptide bonds between the amino acids.

• A genetic code is a sequence of base triplets which codes for specific amino acids.
• In a genetic code, each base triplet is read in sequence, separate from the triple before and after it – the code in non-overlapping.  
• The genetic code is also degenerate – there is more possible combination of triplets than amino acids. This means that some amino acids are coded for by more than one base triplet.  
• Some triplets are used to tell the cell when to start and stop production of the protein – these are called start and stop signal/codons. They’re found at the beginning and end of the gene. (example – UAG is a stop signal)
• The genetic code is also universal – the same specific base triplet codes for the same amino acids in all living things. 

First stage of protein synthesis: 

-Gene is unwound and unzipped.

-Free RNA nucleotides pair up with unpaired bases on template strand.

-Temporary bonds formed, forming the mRNA strand.

-This strand disconnects from DNA and leaves through nuclear pore in the nucleus to be translated.

-The DNA then re-forms and coils back into a double-helix.

Second stage of protein synthesis:

-The mRNA then attaches to the ribosome.

-The ribosome moves along the mRNA.

-The tRNA molecule will attach to the mRNA with its anticodon that is complimentary to the codon on the mRNA.

-As each tRNA brings an amino acid, peptide bonds form between the amino acids forming a polypeptide chain.

-Then the tRNA molecule moves away leaving the amino acid behind.

-The polypeptide chain then moves away from the ribosome and translation is complete.