TRANSCRIPTION & TRANSLATION Topic 3.5 IB Biology Miss Werba

CARBS, LIPIDS & PROTEINS 3.5 TRANSCRIPTION & TRANSLATION TOPIC 3 - BIOCHEMISTRY 3.1 ELEMENTS & WATER 3.2 CARBS, LIPIDS & PROTEINS 3.3 DNA STRUCTURE 3.4 DNA REPLICATION 3.5 TRANSCRIPTION & TRANSLATION 3.6 ENZYMES 3.7 CELL RESPIRATION 3.8 PHOTOSYNTHESIS J WERBA – IB BIOLOGY 2

THINGS TO COVER Structure of DNA vs RNA Process of transcription Codons Process of translation Relationship between one gene and one polypeptide J WERBA – IB BIOLOGY 3

COMPARING DNA & RNA DNA = DEOXYRIBONUCLEIC ACID DNA is found in the nucleus DNA is made up of nucleotides Each nucleotide consists of: a nitrogenous base  T, C, A or G a pentose sugar  deoxyribose a phosphate group There are four DNA nucleotides It is double stranded and twisted into a double helix A T C G D

COMPARING DNA & RNA RNA = RIBONUCLEIC ACID RNA is found both inside and outside of the nucleus RNA is also made up of nucleotides Each nucleotide consists of: a nitrogenous base  U, C, A or G a pentose sugar  ribose a phosphate group There are four RNA nucleotides It is single stranded and therefore linear A U C G R

COMPARING DNA & RNA FEATURE DNA RNA Number of strands 2 strands 3.5.1 COMPARING DNA & RNA FEATURE DNA RNA Number of strands 2 strands 1 strand Sugar deoxyribose ribose Bases A=T C≡G Nucleotides may contain T A=U C≡G Nucleotides may contain U Shape Double helix Linear Check whether question is asking for differences between molecules or nucleotides. You cannot say that a nucleotide is double stranded! J WERBA – IB BIOLOGY 6

COMPARING DNA & RNA Base pairing RNA is used to copy the coded information from the DNA so that it can utilised by the cell. This occurs for two reasons: So that the DNA remains protected within the nucleus and doesn’t get damaged whenever it is used The DNA is too big to fit through the nuclear pores (gaps) in the membrane surrounding the nucleus, while RNA is small. DNA and RNA both follow Chargaff’s base pairing rule HOWEVER, in RNA, Uracil pairs with Adenine (instead of Thymine)

COMPARING DNA & RNA Base pairing U T D R

TYPES OF RNA – bonus  Messenger RNA (mRNA) 3.5 TYPES OF RNA – bonus  Messenger RNA (mRNA) Copy of gene for translation Transfer RNA (tRNA) Brings amino acid to ribosome Ribosomal RNA (rRNA) Catalytic part of the ribosome J WERBA – IB BIOLOGY 9

3.5.2 DNA TRANSCRIPTION Definition: Process of making an RNA copy of a DNA sequence (gene) Location: nucleus Requires: DNA, RNA nucleotides, RNA polymerase, ATP J WERBA – IB BIOLOGY 10

3.5.2 DNA TRANSCRIPTION RNA polymerase binds to a promoter sequence and separates the DNA strands RNA nucleotides (ribonucleotides) are brought in to pair with the exposed DNA bases. The complementary base pairing rule is followed: An exposed C base  G base will be brought in An exposed G base  C base will be brought in An exposed T base  A base will be brought in An exposed A base  U base will be brought in J WERBA – IB BIOLOGY 11

3.5.2 DNA TRANSCRIPTION RNA polymerase joins the RNA nucleotides together with covalent bonds The transcription process stops when a termination sequence is reached The mRNA then leaves the nucleus and heads to the cytoplasm through the nuclear pores. Transcription is necessary b/c: DNA is too precious to be moved all the time DNA is too big to exit through the pores J WERBA – IB BIOLOGY 12

TRANSCRIPTION Copying the code Control-click on the picture to follow the hyperlink!

THE GENETIC CODE Codons: 3.5.3 THE GENETIC CODE Codons: Triplet of bases on the mRNA which encodes a particular amino acid There are 4 bases  therefore 43 =64 combinations The order of the codons determines the amino acid sequence for a protein The coding region always starts with a START codon (AUG) and terminates with a STOP codon J WERBA – IB BIOLOGY 14

3.5.3 THE GENETIC CODE J WERBA – IB BIOLOGY 15

THE GENETIC CODE Features of the genetic code: Universality 3.5.3 THE GENETIC CODE Features of the genetic code: Universality (Almost) every living thing uses the same code Degeneracy Multiple codons may code for one amino acid J WERBA – IB BIOLOGY 16

3.5.4 DNA TRANSLATION Definition: Process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain Location: cytoplasm (at ribosomes) Requires: mRNA, ribosome, tRNA, amino acids, ATP J WERBA – IB BIOLOGY 17

3.5.2 DNA TRANSLATION Remember: MR CATAP (mRNA, ribosome, codon, anticodon, tRNA, amino acid, polypeptide) mRNA binds to a ribosome which initiates translation The mRNA is read in codons (from start codon = AUG) Anticodons on tRNA align opposite appropriate codons (according to complementary base pairing) tRNA brings amino acids to the ribosome (according to the genetic code) Ribosomes join amino acids together with peptide bonds When a stop codon is reached translation stops and a polypeptide chain is released J WERBA – IB BIOLOGY 18

3.5.4 DNA TRANSLATION J WERBA – IB BIOLOGY 19

ONE GENE, ONE POLYPEPTIDE 3.5.5 ONE GENE, ONE POLYPEPTIDE Gene: a segment of DNA coding for a polypeptide Proteins may be made up of multiple polypeptides and therefore can be made from multiple genes (e.g. haemoglobin) One gene gives rise to one polypeptide! J WERBA – IB BIOLOGY 20

TRANSLATION Building the protein Control-click on the picture to follow the hyperlink!

Sample questions Q1. What sequence of processes is carried out by the structure labeled X during translation? Combining with an amino acid and then binding to an anticodon Binding to an anticodon and then combining with an amino acid Binding to a codon and then combining with an amino acid Combining with an amino acid and then binding to a codon J WERBA – IB BIOLOGY 23

Sample questions A1. D J WERBA – IB BIOLOGY 24

Sample questions Q2. Explain the significance of complementary base pairing for replication, transcription and translation. (8 marks) J WERBA – IB BIOLOGY 25

Sample questions A2. A-T and C-G in DNA; A-U and C-G in RNA; complementary base pairing in replication ensures identical nucleotide sequence of new complementary strands; semi-conservative replication; transcription produces RNA sequence complementary to the DNA sequence (of the gene); triplets of nucleotides on mRNA are codons; translation converts mRNA sequence of information into a specific amino acid chain (polypeptide); (each class of) tRNA carries a specific triplet of (three) bases called an anticodon; anticodons bind to codons by complementary base pairing; (each class of) tRNA with specific complementary anticodons carry specific amino acids; sequence of mRNA codons translates into specific amino acid sequence; enables conservation of information transfer from DNA to RNA to polypeptide; J WERBA – IB BIOLOGY 26