UNIT 5 BIOLOGY: GENETIC CONTROL OF PROTEIN STRUCTURE AND FUNCTION

• each amino acid in a proteim is coded for by a sequence of three nucleotide bases - codon 
• the code is degenerate - most amino acids have more than one codon e.g amino acid leucine has 6 codons 
• stop codons - whcih mark the end of polypeptide chain and dont code for an amino acid 
• non overlapping - 
• universal code - same codons code for same amino acid in all organisms 

RNA IS A POLYMER MADE UP OF REPEATING MONO NUCLEOTIDE SUB UNITS - POLYNUCLEOTIDE 

IT FORMS A SINGLE STRAND IN WHICH EACH NUCLEOTIDE IS MADE UP OF A:

• PENTOSE SUGAR - RIBOSE (not deoxyribose like dna)
• ONE OF ORGANIC BASES (uracil replaces thymine as a base) 
• PHOSPHATE GROUP 

2 TYPES OF RNA, THAT ARE IMPORTANT IN PROTEIN SYNTHESIS:

MRNA AND TRNA 

SINGLE POLYPETIDE STRAND 

MRNA IS MADE IN NUCLEUS DURING TRANSCRIPTION 

IT CARRIES GENETIC CODE FROM THE DNA OUT OF NUCLEUS VIA PORES IN THE NUCLEAR ENVELOPE AND ENTERS CYTOPLASM 

RIBOSE SUGAR 

BASES = A U G C 

SINGLE POLYNUCLEOTIDE STRAND 

FOLDED INTO CLOVER SHAPE 

HYDROGEN BONDS BETWEEN SPECIFIC BASE PAIRS HOLD MOLECULE IN SHAPE

ONE END OF CHAIN EXTEND BEYOND THE OTHER WHERE AMINO ACIDS ATTACH 

AT THE OTHER END IS A SEQUECENE OF 3 BAASES KNOWN AS AN ANTICODON 

FOUND IN CYTOPLASM WHERE IT IS INVOLVED IN TRANSLATION - CARRIES AMINO ACIDS THAT ARE UED TO MAKE PROTEINS TO THE RIBOSOMES 

TRANSCRIPTION - process of making pre-mrna from DNA template 

PROCESS:

• enzyme dna helicase acts on specific region of dna and breaks hydrogen bonds - causing two strands to seperate and exposing nucleotide bases 
• enzyme rna polymerase moves along one of the two dna strands - known as template strand - causing the nucleotide on this strand to join with individual complementary nucleotides from pool present in nucleus 
• as rna polymerase adds the nucleotide one at a time to build a strand of pre-mrna the dna strands rejoin behind it 
• when rna polymerase reachees a stop codon it detatches and production of pre mrna is complete

EXON - CODES FOR PROTEINS 

INTRON - DOES NOT (interfere with syntheisis of a polypeptide) - removed in a process called splicing

once pre-mrna is spliced its called mrna 

mrna molecules too large to diffuse out of nucleus leaves through nuclear pore - outside in cytomplasm it becomes attatched to ribosomes ready for translation 

PROCESS:

• A ribosome becomes attached to the starting codon at one end of mRNA molecule 
• tRNA molecule with complementary anticodon sequence moves to ribosome and pairs up with sequence on mRNA through specific base pairing - this tRNA carries an amino acid 
• a second tRNA molecule attatches itself to the next codon on the mRNA in the same way 
• the two amino acids attached are joined by peptide bond 
• a third trna molecule binds to the next codon on mrna 
• as this happens , the first trna is released from amino acid and is free to collect another amino acid from amino acid pool in cell 
• process continues porducing a polypeptide chain until a stop codon is reached 

CHANGE TO QUANTITY OR STRUCTURE OF DNA - MUTATION 

CHANGE TO ONE OR MORE NUCLEOTIDE BASES IN DNA IS CALLED GENE MUTATION 

TWO MAIN TYPE OF GENE MUTATION:

• SUBSTITUTION OF BASES 
• DELETION OF BASES 

THEY TYPE OF GENE MUTATION IN WHICH A NUCLEOTIDE IN A DNA MOLECULE IS REPLACED BY ANOTHER NUCLEOTIDE WITH A DIFFERENT BASE 

A CHANGE TO A SINGLE BASE COULD RESULT IN:

• NONSENSE MUTATION - if base change results in the formation of one of three stop codons that mark end of polypeptide chain - final prtoein would be different and not be able to perform its function correctly 
• MIS SENSE MUTATION - when base change results in different amino acid being coded for - polypeptide chain produced will differ in a single amino acid 
• SILENT MUTATION - when substiuted base still codes for same amino acid as before 

WHEN A NUCLEOTIDE IS LOST FROM THE DNA SEQUENCE - amino acid sequence of polypeptide will be entirely different - always read in units of three and so it shifts them along - frame shift - read in wrong three base groups 

MORE DAMAGING AT START OF SEQUENCE ALTERS MORE 

LESS IMPACT NEAR END OF SEQUENCE 

GENE MUTATIONS ARISE DURING DNA REPLICATION 

MUTATION INCREASED RATE FROM OUTSIDE FACTORS KNOWN AS MUTAGENIC AGENTS:

• HIGH ENERGY RADIATION - DISRUPT DNA MOLECULE 
• CHEMICALS THAT ALTER DNA STRUCTURE OR INTERFERE WITH TRANSCRIPTION 

COSTS AND BENEFITS:

• GOOD - PRODUCE GENETIC DIVERSITY FOR NATURAL SELECTION 
• BAD - PRODUCE ORGANISM LESS WELL ADAPTED TO ENVUIRONMENT - CAN ALSO AFFECT NORMAL CELLULAR ACTIVITES SUCH AS CELL DIVISOION.

CELL DIVISION CONTROLLED BY GENES - TWO EXAMPLE:

• PROTO - ONCOGENES -stimulate cell division 
• TUMOUR SUPRESSOR GENES - slow cell division 

A GENE MUTATION CAN CAUSE PROTOONCOGENES TO TURN INTO ONCOGENES WHICH CAN AFFECT CELL DIVISION....

• RECEPTOR PROTEIN ON CELL SURFACE CAN BE PERMANENTLY ACTIVATED - SO CELL DIVISION IS SWITCHED ON EVEN IN THE ABSCENCE OF GROWTH FACTORS 
• IT MAY CODE FOR GROWTH FACTORS THAT IS THEN PRODUCED IN EXCSSIVE AMOUNTS 

CELLS DIVIDE TOO RAPIDLY AND A TUMOUR OR CANCER DEVELOPS 

OPPOSITE ROLE TO PROTONCOGENES - THEY INHIBIT CELL DIVISION 

IF IT BECOMES MUTATED IT IS INACTIVATED - CELL DIVISION THERFFORE INCREASES 

MOST MUTATED CELLS IDE HOWVER SOME CAN MAKE CLONES OF THEMSELVES AND FORM TUMORURS 

NOT ALL TUMOURS ARE HARMFUL (MALIGNANT) SOME ARE HARMLESS (BEGNINGN)