TOPIC 3: DNA AND RNA (GENETICS)
DNA
• Deoxyribonucleic acid
• Made up of two strands
• There are four nitrogenous bases present in the middle of the DNA structure
• Adenine links with Thymine (A and T)
• Guanine links with Cytosine (G and C)
• These are known as complementary base pairs as they always link together by hydrogen bonds
• The hydrogen bonds between each complementary base pair holds the two strands of the DNA together
Nucleotides
• There are four nucleotides present in the DNA structure
• Each nucleotide contains:
● A phosphate
● A five-carbon sugar known as ribose
● One of the four nitrogenous bases.
Polynucleotide
• A group of nucleotides bonded together • Purines and Pyrimidines always bond together ● Adenine isa purine and bonds with the pyrimidine thymine ● Guanine isa purine and bonds with the pyrimidine cytosine
DNA shape
• The two strands of the DNA are attached by the hydrogen bonds between the complementary base pairs
• The two strands are wound round each other to form the DNA double helix
RNA
• Ribonucleic acid
• RNA also contains four complementary bases ● Adenine links to Uracil (A and U)
● Guanine links to Cytosine (G and C)
• RNA isa single stranded molecule
• RNA can move in and out of the nucleus into the cytoplasm • RNA copies the genetic code/sequence of bases on a DNA strand and carries it out of the nucleus to the ribosomes
for protein synthesis Messenger RNA (mRNA)
• Copies the sequence of bases ona DNA strand using the complementary RNA bases from the cytoplasm • Is made up of codons •A codon is three bases that will code for an amino acid
• There are three types of codon present in mRNA ● A start codon
● A group of codons that form a protein
● A stop codon • RNA polymerase links together the bases on the mRNA strand • mRNA brings the sequence out of the nucleus to the ribosomes to make a protein
Ribosomal RNA (rRNA)
• Is found in the ribosomes
• Attracts mRNA to the ribosome
• Bonds to mRNA and holds it in the ribosome for protein synthesis Transfer RNA (tRNA) • Are found in the cytoplasm of the cell • Each tRNA ● Has an amino acid attached ● Has three bases attached known as an anticodon • The three bases that form the anticodon on tRNA will complementary base pair with the three codons on the mRNA in the ribosome • The amino acids on the different tRNA molecules will bond together to form a protein sequence
Steps in protein synthesis
Transcription is the copying of bases from DNA to mRNA • Transcription occurs in the nucleus • mRNA bases copy the DNA sequence • mRNA moves out of the nucleus towards the ribosome • In the ribosome mRNA bonds with rRNA Translation is the conversion of a sequence of bases on mRNA into a sequence of amino acids in a protein • Occurs in the ribosome • The mRNA strand is pulled through the ribosome • The start codon begins protein synthesis • Each codon on mRNA attracts an anticodon on tRNA • The tRNA carries an amino acid to the ribosome as its anticodon bonds with the mRNA codon • The amino acids pulled to the ribosome by tRNA bond together to form a sequence • This sequence of amino acids forms a protein
• During the final stage, the protein is folded so it can carry Part (b) deals with protein synthesis
(b)
(i) Explain the terms transcription and translation Transcription: The copying of bases from DNA to mRNA (3) Translation: The conversion of a sequence of bases on mRNA into a sequence of amino acids in a protein (3)
(ii) In which structures in the cell does translation occur? Ribosomes (3)
(iii) How many bases in sequence make up a codon in mRNA?
Three (3)
(iv) Each mRNA codon specifies one of three possible outcomes during protein synthesis
Name these three possible outcomes
Start codon (3)
Sequence of codons used to make a protein (3) Stop codon (3)
(v) What does the letter ‘t’ stand for in tRNA? Transfer (3)
(vi) During translation one end of a tRNA molecule attaches to an mRNA codon.
What is usually attached to the other end of the tRNA molecule? An amino acid (3)