Nucleic Acids

Made up of a phosphate group, a pentose sugar and an organic nitrogenous base

pyrimidine bases are a single ring and are thymine, cytosine and uracil

purine bases are double ring and are Adenine and Guanine

Deoxyribonucleic acid

very stable and large molecule. chromatin in the nucleus, small amounts in the mitochondria and chloroplasts

Self-replication and contains the code for a protein

made up of a deoxyribose sugar, a phosphate group. Adenine, Guanine, Cytosine and Thymine.

Has antiparallel strands

nucleotides are held by bonds between the phosphate group attached to the 5th carbon atom on one pentose sugar and the 3rd carbon atom of the other

5' prime end to 3' prime end

2 sugar-phosphate backbones protect the genetic information in a sequence of bases that face each other

the bases are held by hydrogen bonds

adenine and Thymine have 2 hydrogen bonds and Guanine and cytosine have 3 hydrogen bonds

DNA must replicate together, so each daughter cell receives an exact copy o the genetic information.

Hydrogen bonds holding complementary base pairs break

DNA helicase breaks the H bonds and separates the 2 strands.

Each DNA strand acts as a template

free nucleotides align opposite complementary bases

DNA nucleotides catalyses condensation reaction between 2 DNA nucleotides

DNA polymerase catalyses condensation reaction between 2 DNA nucleotides

the new molecule is made from 1 original template strand and 1 newly synthesised strand

Semiconservative replication

-         Conservative: The original parental double helix remains intact, and a whole new DNA molecule is made

-         Semi-Conservative: Each strand in the parental double helix acts as a template to synthesise a new polynucleotide strand. Each new DNA molecule contains one template strand and one newly replicated strand

-          Dispersive: Each new DNA molecule contains fragments of both the parental double helix and newly synthesised DNA

1.      E.coli was grown in a medium containing amino acids made with 15N.  The bacteria produced nucleotides containing 15N.  All the bacteria’s DNA contained 15N.The DNA was  extracted and the suspension then centrifuged

2.      The bacteria were washed and then transferred to a medium containing amino acids with 14N and allowed to divide once. The DNA was extracted and the suspension then centrifuged.

3.      The bacteria were allowed divide again on the 14N medium.  Again, the DNA was centrifuged.

•           After one generation, semi-conservative replication would give one band in the middle
•        After one generation, conservative replication would not give a band in the middle, only a band at the top made from N14 and a band at the bottom made from N15
•          Dispersive replication would only produce one band, not two which got progressively higher in the tube

• Ribonucleic acid
• Single-stranded polynucleotide with Adenine, Guanine, Cytosine, Uracil and a Ribose Sugar
•  RNA is a short-lived molecule, found mainly in the cytoplasm of the cell but also in the nucleus.

-          DNA stores genetic information coded in the sequence of bases

-          The sequence of bases determines the sequence of amino acids that are joined together to form a polypeptide

-          Some proteins contain more than one polypeptide. A section of DNA that codes for a specific polypeptide is called a gene; this is called the one gene-one polypeptide hypothesis.

-          Three bases encode each amino acid. This is called the Triplet code

-          There are 64 possible codes but only 24 amino acids found in proteins.

-          Each amino acid has more than one triplet code, the code is then called degenerate.

-          Some triplet codes do not code for amino acids but are ‘stop codons’ and mark the end of translation

-          The genetic code is Universal: the same triplet codes for the same amino acid in all living organisms

-          The genetic code is non overlapping each base occurs in only one triplet

• In Transcription, it happens in the nucleus, the genetic code for a specific protein is copied. a complementary strand of mRNA is formed from one template strand of DNA. mRNA leaves the nucleus via the nuclear pore to the ribosome.
• in Translation, it happens in the ribosome. the genetic code is translated into a polypeptide. amino acids that correspond to the codons on the mRNA are brought to the ribosome by tRNA. the amino acids are joined together to form a polypeptide chain

• DNA Strand is used as template strand to form a mRNA.
• DNA helicase breaks the H bonds holding the base pairs in a specific region of the DNA causing the 2 strands to unwind
• RNA Polymerase binds to the template strand at the beginning of the sequence to be copied.
• free RNA nucleotides align opposite their complementary bases on the template strand
• RNA Polymerase catalyses the addition of RNA nucleotides until it reaches a stop codon
• behind the RNA polymerase, the DNA strands rewind
• mRNA leaves via the nuclear pore and travels to the cytoplasm

• The initial mRNA molecule is longer than the final mRNA translated at the ribosome
• the initial version is the pre mRNA which needs sequences of bases to be removed
• introns are non-coding nucleotide sequences that are removed after the transcription by the endonuclease
• exons are coding sequences left behind and spliced together by ligase enzymes to form the final mature mRNA.
• prokaryotic DNA does not contain introns

tRNA Molecule:

• cloverleaf shape
• carries specific amino acids to the ribosome
• anticodon determines which amino acid the molecule carries
• attaches to the amino acid in the cytoplasm. requires ATP and called Amino acid activation

Ribosome:

• made from protein and rRNA.
• free in the cytoplasm or attached to rough ER
• consists of large and small subunit
• large subunit has two attachment sites for tRNA molecules
• smaller subunit binds to the mRNA

Initiation

• Ribosome attaches to start codon on the mRNA
• 1st tRNA binds to the 1st attachment site, the anticodon joins to the complementary codon on the mRNA to form a codon-anticodon complex
• 2nd tRNA forms a codon-anticodon complex at the 2nd attachment site

Elongation

• the ribosomal enzyme catalyses the formation of a peptide bond between adjacent amino acids
• 1st tRNA leaves site 1 and returns to the cytoplasm
• the ribosome moves down the mRNA one codon, the 2nd tRNA moves from site 2 to 1
• new tRNA binds to site 2

Termination

• the sequence repeats until a stop codon is reached
• ribosome- mRNA-polypeptide complex separates

Polysome - the complex of several ribosomes on one mRNA that can move along one mRNA strand at one time.

• a sequence of codons on the mRNA determines the sequence of amino acids
• primary structure of the polypeptide
• can then be transported to the Golgi body in vesicles for further modification
• can be folded or chemically modified by a combination of non-proteins to make glycoproteins, lipoproteins and phospho-proteins

• mRNA transcripted from DNA strand in the nucleus
• mRNA then travels to the RER to be translated to a polypeptide
• the polypeptide travels to the Golgi body in a vesicle for further modification
• it then travels to the cell membrane in a vesicle (exocytosis)

• Adenosine Triphosphate
• Contains Adenine, ribose sugar and Guanine, Uracil, Adenine, Cytosine
• Respiration oxidises glucose in a series of small reactions to release energy in the form of ATP.
• ATP is an energy source, it is synthesised as it is required
• involved in energy changes, carries energy to where it is needed and releasing energy when it is broken down

Roles of ATP

• Muscle Contraction
• Active Transport
• Cell Propulsion
• Chromosomal Activity
• Synthesis of Macromolecules

-          ATP Synthase combines ADP and a pi in a condensation reaction

-          This requires an input of energy so is an endogernic reaction. requires 30.6 kJ mol-1 and the addition of the phosphate is phosphorylation.

-          ATPase hydrolyses the terminal phosphate bond releasing a small 30.6kjmol in an exergonic reaction to form ADP and PI.

Properties of ATP

-          Only one enzyme is needed to release energy from the breaking of one high energy bond

-          ATP releases energy in small amount when and where it is needed

-          ATP is the universal energy currency in all cells in all organisms

-          ATP is easily transported across membranes