Cell replication, transcription and translation Flashcards
DNA nucleotide
DNA subunits consist of one nitrogenous base, one deoxyribose and one phosphate group.
DNA Coiling
DNA wound around histone protein to form nucleosome (contains 140-150 bases and 20-60 bases spacing in between each). Six nucleosome combined and produces a solenoid which then converts into chromatin loop (contains 100,000 base pairs or 100 kilo base) attached to protein scaffold.
Replication fork
Where DNA synthesis begins
What does Replisome contains?
DNA polymerase: add complementary nucleotide and does the proof reading of newly added nucleotide
DNA Helicase: unwinding of DNA strands
RNA primase: produces RNA primer
DNA replication problems and rectification
- Supercoiling: topoisomerase enzyme (DNA Gyrase) prevents this by negative supercoiling
- How much DNA to unwound: unwinding small sections at a time - leading and lagging strands
- preventing binding of single strands: Single strand base protein will prevent this.
Leading strand
Replication occur in one continuous strand and it is called leading strand
Lagging strand and Okazaki fragment
Replication occurs in a small sections. Starts by RNA primer and then DNA polymerase adding bases to the strand. This single small section is called Okazaki fragments and this strand is called lagging strand
Replication bubble
it allows DNA replication to occur at multiple locations on chromosome to speed up the replication process
Composition of RNA strand in contrast to DNA
RNA: contains ribose sugar, and uracil nitrogenous base and is a single strand
Role of RNA primer in replication
It provides -OH group to DNA strand for DNA polymerase to start to add nucleotide at 3’ end. -OH group also forms a phosphodiesterase bond with alfa-phosphate of a nucleotide.
Enzymes for replication in lagging strand and removing RNA primer
Ribonuclease H - removes RNA primer
DNA polymerase delta - adds DNA in place of RNA primer
DNA ligase - joins the small sections of lagging strand to complete a continuous single strand
Transcription process
Producing mRNA from DNA
1. RNA polymerase II binds to promoter region
2. Base addition on 3’ and transcription proceeds in 5’ to 3’
3. 5’ cap - to prevent degradation of mRNA strand - capping with guanine nucleotide which also serve the starting point for translation
4. poly-A-tail - When termination sequence appears on DNA strand. addition on 100-200 adenine bases on 3’ end of mRNA produces poly-A-tail. Also prevents degradation of mRNA in cytoplasm.
5. Detachment of DNA, mRNA and RNA polymerase - produce primary mRNA
6. Splicing of introns (non coding regions) and joining of exons (coding regions) produce mature mRNA that travels to cytoplasm for translation.
Euchromatin vs heterochromatin DNA and epigenetic modifiers
DNA strand wound loosely around histone protein - called switched on - available for transcription - condensed and acetylated by methyl group - called euchromatin
DNA wound tightly, not available for transcription, switched off, decondensed and hypoacetylated - called heterochromatin
Epigenetic modifier - methylation of promoter region, microRNA binding to promoter region and acetylation of histone leads to modify expression of gene but does not affect DNA sequence
Main regions of promoter region
- initiation box: where transcription starts
- TATA box: A-T rich sequence - easier separation of DNA by RNA polymerase II
- Enhancer sequence: of DNA binds to transcription factors to initiate transcription
Translation
mRNA translate into polypeptide chain (protein)
Occurs on ribosome in cytoplasm.
Ribosome contains rRNA that binds mRNA and tRNA to ribosome.
Ribosome binds to initiator site (AUG-Methinone) on mRNA and then to tRNA (contains 80 nucleotide)
tRNA has 3’ amino acid binding site - AA join via covalent bonding
Opposite to 3’ site it has anticodon - for base pairing of codon of mRNA
Ribosome reads the codons on mRNA from 5’ to 3’ direction and add AA on polypeptide chain.
This process continues until stop codon is reached which are UAA, UGA, UAG and process terminates after.
On final AA polypeptide chain -NH2 of AA becomes 5’ and -COOH group becomes 3’
3 sites of ribosome
Acceptor site, peptide site and exit site.
At any given time only 2 tRNA binds to ribosome
3 STOP codons
UGA, UAA, UAG
Central Dogma in genetics
progression from DNA to mRNA to protein
MicroRNA
RNAs that do not convert into the proteins and contain small chain of nucleotides (17 to 27). Eg: miRNA, longchainRNA (lncRNA), Small interfering RNA (siRNA) - use in cancer and gene therapies.
What is consensus sequences
Enzymes that carry out splicing of introns are directed to the appropriate locations by DNA sequences known as consensus sequences (so named because they are common in all eukaryotic organisms), which are situated adjacent to each exon.
Monocistronic mRNA vs polycistronic mRNA
Monocitronic mRNA - in eukaryotic cells. Gene –mRNA–prtotein
Polycistronic mRNA - in Prokaryotic cells. Cluster of Genes (operons) –mRNA—multiple proteins
