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Nucleotide structure

A nucleotide has 3 components

A phosphate group         a pentose sugar (ribose in RNA, deoxyribose in DNA)    Nitrogenous base

There are 2 groups of organic bases:

  • Pyrimadine- Thymine, Cytisine and Uracil
  • Purines- Guanine and Adenine
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ATP and an energy carrier and its structure

adenine-ribose-phosphate x 3     remove 1 prosphate= 30kj released

  • Roles of ATP- MAMS
  • Metabolic processes
  • Active Transport
  • Movement- muscle contraction
  • Secretory-packaging and transport

As a supplier;

  • Small amounts of enery are released, not too much
  • Only one enzyme is neededto release energy
  • provides anergy for most chemical reactions
  • doesnt release too much energy as heat

the addition of phosphate to ADP is called phosphorylation.

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The Structure of DNA

DNA is composed of two polynucleotide strands wound around eachother in a double helix

The deixyribose sugar and phosphate groups are on the outside of the DNA molecule and form the 'backbone'

The bases of the two strands face inwards, a-t and c-g, hydrogen bonds connect them and they form complementary base pairs and maintain the shape of the helix.

the nucleotides on one strand are arranged in the opposite direction from those on the complementary strand, this is known as ANTIPARALLEL

DNA is suited to its function as:

  • it is a very stable molecule which remains unchanged from generation to generation
  • it is a large molecule which carries a large amount of genetic information
  • the two strands can separate as they are held together with h-bonds
  • as the base pairs are on the inside, the information os protected.
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The structure of RNA

  • RNA is a single stranded polynucleotide

There are three types pf RNA which are involved in processes of protein synthesis:

1. Messenger RNA

Is a long single stranded molecule. it is synthesised in the nucleusand carries the genetic code from the DNA to the ribosomes in the cytoplasm

2. Ribosomal RNA

Is found in the cytoplasm and comprises of large complex molecules. ribosomes are made from r-RNA and protein. They are the site of translationof the genetic code into protein.

3. Transfer RNA

small single stranded molecule, the shape is described as a clover leaf. tRNA transports a sequence of three bases called the anti codon and transports amino acids to the ribosomes in protein synthesis.

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Functions of the DNA

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Functions of the DNA

DNA has two main roles

1. Replication

DNA comprises of two complementary strands, the base sequence of one strand determining the base sequence of the other. If the strands are separated, two identical helces can be formed and each parent strand acts as a template.

2. Protein synthesis

the sequence of bases represents the information carried in DNA and determines the sequence of amino acids in proteins

Semi-conservative replication- mode of DNA replication in which each strand of a parental double helix acts as a template for the formation of the new molecule, each containing and original strand and a newly synthesised complementary daughter strand.

The Meselson- Stahl experiment

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The Genetic Code

The Genetic Code is a Triplet code

  • Biochemical experiments show that a polynicleotide strand always had three times the no. of bases than the amino acid chain it coded for
  • If three bases were removed from the chain, there would be one fewer amino acid.

This suggests that 3 bases code for one aminoacid, this result was supported by the logic of arithmetic, there are 4 differen bases and over 20 amino acids

if two bases coded = 4 squared= 16, which isnt enough therefore, must be 4 cubes= 64

Characteristics of the genetic code

  • It is a triplet code
  • more than one triplet can encode each acid so the code is described as 'redundant'
  • The code is punctuated with 'stop' codons which mark the end of a portion being translated
  • it is universal to all organisms

Codon= Triplet pf bases in m=RNA that codes for a particular amino acid, or a punctuation sign.

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Exons and Introns

DNA contains information for making polypeptides. An RNA version is first made from the DNA, this RNA has to be processed before it can be used to synthesise the polypeptide.

The initial RNA is much longer than the m-RNA and is somethimes called pre-mesenger. The sequences removed are called introns and the sequences left are exons.

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Protein Synthesis

Here are the stages of protein synthesis;

1. Transcription

One strand of DNA acts as a template for the production of mRNA. This occurs in the nucleus.

2. Translation

the mRNA acts as a template to which complementary tRNA molecules attach, and the amino acids they carry are linked to form the polypeptide. This occurs on ribosomes in the cytoplasm.

DNA                                          mRNA                                          POLYPEPTIDE

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DNA does not leave the nucleus,  transcription is the process whereby the gene acts as the template for the production of mRNA , which carries the information to the cytoplasm

1. The enzyme DNA helicase breakes the hydrogen bonds and unwinds the DNA.

2. The enzyme RNA polymerase binds to the template strand of DNA at the begining of the sequence to be coppied

3. Free RNA nucleotides align opposite the template strand, based on the complementary relationship.

4. RNA polymerase moves along the DNA forming bonds that add the free RNA nucleotides one at a time to the growing strand. This results in the synthesis of mRNA alongside the unwound DNA. Behind the RNA polymerase the DNA rewinds.

5. The RNA polymerase separates from the strand when it reches the stop codon.

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In translation the strand od mRNA is used to code for a specific sequence of amino acids, forming a polypeptide. It takes place on a ribosome and involves tRNA

Each ribosome is made up of two sub-units:

  • The larger sub unit has two sites for the attachment of tRNA molecules
  • The smaller subunit binds to the mRNA

The ribosome acts a framework holding the codon and anticodon together untill the amino acids bind. The ribosome moves along the mRNA adding one amino acid at a time untill a polypeptide chain is assembled.

Translation ocurs as follows;

Initiation- start codon

Elongation- the two aminos are suficiently close to form a peptide bond

Termination- stop codon

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Genes and Polypeptides

(energy is needed in the form of ATP for the amino acid to attach to the tRNA)

One gene-one polypeptide! as many genes contain more than one polypebtide chain

Post translation modification

Polypeptides can be combined with :

  • Carbohydrate, making glycoproteins
  • Lipids, making lipoproteins
  • Phosphate, making phospho-proteins

Heamoglobin is highly modified, each polypeptide has alpha helix regions folded into a tertiary structure. Four polypeptides are combined. In addition it is combined with four haem groups.

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