DNA & Genomics Flashcards
5’ cap
[structure]
- addition of 5’ methyl guanosine nucleoside triphosphate to the first nucleotide by 5’-5’ triphosphate linkage, forming 5’ methylguanosine cap
- addition of 5’ methylguanosine cap is catalysed by guanylyl transferase
- 5’ cap is added after transcription of the First 20-40 nucleotides
[function]
- facilitates the export of mature mRNA from nucleus into cytoplasm
- protects mRNA from 5’ exonucleases, thus confers stability to the mRNA
- facilitates in binding of ribosomes to mRNA
3’ poly-A tail
[structure]
- polyadenylation signal sequence, AAAAAU,is a signal for transcription termination and polyadenation of the 3’ end of mRNA
- 200 adenine residues are added to the 3’ end of the pre-mRNA, forming the 3’ poly-A tail
- formation of poly-A tail is catalysed by poly-A polymerase
[function]
- facilitates the export of mature mRNA from nucleus into the cytoplasm
- slows down degradation by 3’ exonucleases; the longer the poly-A tail, the longer the half life of mRNA
Difference in structure between ribosome and RNA polymerase
- Ribosome consists of 2 subunits whereas RNA polymerase consists of large molecule of protein with many subunits
- Ribosome is made up of proteins and ribosomal RNA whereas RNA polymerase is made up of protein only
List three ways in which the process of transcription is different from the process of translation
- Transcription uses DNA as a template to synthesise mRNA while translation uses mRNA as template to synthesise polypeptide chains
- Transcription results in the formation of phosphodiester bonds catalysed by RNA polymerase while translation results in the formation of peptide bonds cant alters by peptidyl transferase
- Transcription uses ribonucleotides as monomers to build up into polymers while translation uses amino acids as monomers to build up into polymers
Explain the reason for the production of Okazaki fragments
- DNA is anti parallel as DNA polymerase adds nucleotides in the 5’ to 3’ direction on both sides of the replication fork
- One daughter strand is synthesised towards the fork while the other is synthesised away from the replication fork
- Hence, resulting in the lagging strand being synthesised discontinuously via okazaki fragments
- Multiple primers need to be added to provide each fragment with a 3’ OH end for the addition of bases
Describe how replication of the lagging strand template occurs
- The lagging strand is synthesised discontinuously, via a series of Okazaki fragments away from the replication fork, catalysed by DNA polymerase III
- Each okazaki fragment needs to be primed separately, where primase catalyses the synthesis of the RNA primers
- RNA primers are excised and replaced with deoxyribonucletides by another DNA polymerase
- DNA ligases catalyses the formation of phosphodiester bond between two okazi fragments
Explain why transcription and translation are able to occur simultaneously in a bacterial cell such as E.coli
Bacterial cells do not contain nuclear envelope hence RNA polymerases involved in transcription and ribosomes involved in translation are all found in the cytoplasm
Explain how the information to synthesise polypeptides is coded for by DNA
- Transcription occurs using one strand of DNA as template to synthesise mRNA via complementary base pairing by RNA polymerase
- mRNA is translated by ribosomes to form the polypeptide chain
- Each codon on the mRNA codes for an amino acid, therefore the DNA sequence of a gene determines the primary structure of a polypeptide chain
State what is meant by semi-conservative DNA replication
- The two strands of the parental molecule separate by breaking hydrogen bonds between complementary bases
- Each strand acts as template for synthesis of newly synthesised strand
- The daughter DNA molecule contained one strand which is conserved from the parental molecule and the other newly synthesised strand
