MCB 2: The Central Dogma of Biology Flashcards
What are the two groves on the DNA backbone called?
- major groove:
- most protein-DNA contacts are made in the major groove because the minor is too narrow
- minor grove
What makes the sugar-phosphate backbone charged and polarised?
- phosphate groups in the deoxyribose sugars are negatively charged
Which bases are purine?
How many rings are in a purine base?
- adenine and guanine
- two rings
Which bases are pyrimidine?
How many rings are in a pyrimidine base?
- cytosine and thymine
- 1 ring
What is the Central Dogma of Molecular Biology?
- ‘the two-step process by which the information in genes flows into proteins’
What are the differences and similarities between DNA and RNA?
How is 2m of DNA packaged into a nucleus 10µm in diameter?
Define epigenetics
- a change in gene expression maintained through cell division not involving a change in DNA sequence
- there are three main epigenetic mechanisms
What are the three main epigenetic mechanisms?
Give brief descriptions of them
- DNA methylation:
- the covalent addition of a methyl group to the 5th carbon of a cytosine residue in a CpG dinucleotide
- compared to an on/off switch as the association with gene expression is fairly clearly define
- Histone modifications:
- histones can undergo post-translational modifications on the N terminal tails and affect the way DNA is packaged
- also affects the expression of genes coded by the DNA wrapped around those histones
- Non-Coding RNAs:
- ncRNAs are RNA molecules that are transcribed from DNA but do not encode for proteins
- they perform many critical tasks
- e.g. microRNAs and long non-coding RNAs
Label the basic structure of a gene
Describe how DNA methylation works
- DNA methyltransferases (DNMT) add the methyl-groups to the 5th carbon of the cytosine residue, which is followed by a guanine residue
- this forms the CpG dinucleotide (p = phosphate backbone) - Methyl-Binding Domains (MBD) recognise the methylated CpG and cause downstream effects
- TET proteins act as DNA methylation erasers and removed the methylation mark by oxidising the methylated cytosine
- Methylated promoters prevent transcription machinery from binding and recruits MBDs which recruit other epigenetic modifiers to further silence the gene
Describe how histone modification works
What are non-coding RNAs?
- RNA molecules that are transcribed from DNA but do not encode for proteins
- they perform critical tasks in cells
- microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are key regulators of gene expression
- they can also have enzymatic, structural or regulatory roles
What is miRNA?
Describe briefly its function
- short stretches of RNA that regulate expression of genes post-transcriptionally
- after DNA is transcribed to RNA but before RNA is translated to protein
- compared to a dimmer switch: they fine-tune the level of expression
Explain the biogenesis and function of miRNA
Function:
- silences genes by inactivating messengerRNAs
- primary microRNA is double-stranded
How it works:
- The protein DGCR8 recognises the stem and an enzyme, DROSHA, associates with DGCR8 to form a micro-processor complex into a smaller precursor microRNA
- It is then exported into the cytoplasm to deactivate mRNA of genes
- Then it is exported out of a nuclear pore by the transported molecule Exportin 5
- in the cytoplasm, it is recognised by a large cytoplasm protein called DICER
- DICER cleaves the stem loop and forms a short, double-stranded miRNA molecule
- AGO2 interacts with DICER to bind with the miRNA
- miRNA is unwound and one strand is released
- The remaining strand and AGO2 forms RISC
- It can now be guided to its target by complementary base pairing
- Continue in image
What are long non-coding RNAs (lncRNA) ?
Describe their main roles
