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EDEXCEL (A) SALTERS-NUFFIELD BIOLOGY TOPIC 2 - GENES AND HEALTH NOTES
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Edexcel (A) Salters-Nuffield Biology topic 2 (GENES AND HEALTH) notes.
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TOPIC 2DNA
1. STRUCTURE OF DNA
- DNA is a polynucleotide
- Made up of 2 long strands of mononucleotides twisted around each other to form a double helix
- Strands run in opposite directions = antiparallel
- Nucleotide = contains 3 molecules : deoxyribose, a phosphate group, and nitrogenous base
- 3 molecules linked together by condensation reactions
- Link together by phosphodiester bond in a condensation reaction between deoxyribose of one nucleotide and the
phosphate group of the next nucleotide to form a sugar phosphate backbone
- Bases
- 4 bases
- Adenine (A), Guanin (G), Cytosine ©, Thymine (T).
- A pairs with T
- C pairs with G
2. COMPARE DNA WITH RNA
DNA BOTH RNA
- contains deoxyribose sugar - have a phosphate group - contains ribose sugar (mRNA and
- DNA doesn’t fold - nucleic acids tRNA)
- DNA has thymine, NO uracil - have base guanine - tRNA folds into a clover leaf shape
- found ONLY in NUCLEUS - contain phosphodiester bonds - mRNA and tRNA have uracil instead
- double stranded of thymine
- found in NUCLEUS and
CYTOPLASM (mRNA)
3. STRUCTURE OF tRNA vs mRNA
tRNA mRNA
- folded - straight/unfolded
- has H bonds (holding structure together) - mRNA does NOT have H bonds
- fixed (size/length) - mRNA’s length/size depends on size of gene
- has an anticodon - mRNA has codons
- has an amino acid binding site
4. TRANSCRIPTION
, - DNA molecule unwinds/unzips/uncoils
- DNA helicase
- (RNA mono)nucleotides line up against/attach to one strand/template/antisense strand
- complementary base pairing between DNA and mononucleotides
- RNA polymerase catalyses the formation of phosphodiester bonds in a condensation reaction
- mRNA detaches from DNA
- mRNA leaves through nuclear pore, attaches to a ribosome in cytoplasm
5. TRANSLATION
- mRNA attaches to the ribosome in the cytoplasm
- there are 2 binding sites at the ribosome
- the first tRNA lines up along its ANTICODON along side a complementary codon on the mRNA
- the tRNA is carrying a specific amino acid
- the start codon always codes for the amino acid methionine (AUG)
- hydrogen bonds form between the complementary bases
- a SECOND tRNA arrives at the 2nd binding site at the ribosome
- an enzyme catalyses the formation of a PEPTIDE bond between the two amino acids (peptidyl transferase). The peptide bond is
formed through a condensation reaction
- the ribosome moves along the mRNA to reveal another new codon at the binding site
- the first tRNA returns to the cytoplasm and the process continues until the stop codon is reached
- a polypeptide is formed
- the mRNA detaches from the ribosome
- polypeptide is released into the cytoplasm
6. GENETIC CODE
- made up of 4 bases which create 20 amino acids instead of 4
- genetic code is carried by the DNA which is a triplet base/triplet code
- each of the 3 bases codes for one amino acid
- the code itself is NON-OVERLAPPING
- there are 64 possible 3 letter combinations if 4 DNA bases combine together into a triplet
- 1 triplet sequence is the start code and 3 are stop codes (chain terminators)
- if several triplets can code for the same amino acids, the code is DEGENERATE
7. DIPEPTIDE BOND FORMATION
- 2 amino acids join together when the carboxyl group of 1 amino acid reacts with the amino acid group of the other amino acid
- condensation reaction
- forms a dipeptide with a peptide bond forming between the carbon of the carboxyl group and nitrogen of the amino group
- loss of one molecule of water
8. DNA REPLICATION - semi conservative replication
- DNA helicase catalyses the breakdown of the hydrogen bonds between complementary base pairs
- DNA nucleotides line up opposite the complementary bases in the parent DNA strands and form hydrogen bonds with them
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