Gene Structure Flashcards
what is glycocalyx
a glycoprotein, polysaccharide that surrounds the cell membranes of some bacteria, epithelia and other cells
what are histones
family of basic proteins that associate with DNA in the nucleus and help condense into chromatin
what is UASs
upstream activator sequences
what are eukaryotic gene systems
- transport of stable RNA from nucleus to cytoplasm
- transcription in nucleus and translation in cytoplasm
what is the eukaryotic gene structure like
DNA: associated with histones, linear, genes may contain introns, in nucleus
Internal structures have membrane-bound organelles (mitochondria, chloroplasts, vacuoles)
Ribosomes are 80S
what types of reproduction are there in eukaryotic gene systems
> asexual (mitosis)
sexual (meiosis)
DNA usually paired (diploid or more)
what are the two short sequences in the core promoter structure of prokaryotes
> at -10 (Pribnow box, or -10 element) usually 6 nt: TATAAT, and absolutely essential for transcription start
at -35 (-35 element) usually 6 nt: TTGAGA, is present, transcription is very high
what is the structure of eukaryote gene DNA
Promoter Transcription start 5’ UTR Exons and introns 3’ UTR Transcription end point
what do eukaryote promoters contain
- TATA box (TATAAA) to form RNA polymerase transcription complex
- CAAT box
- Enhancers, silencers and other upstream activator sequences (UASs) differs from those of prokaryotic genes
what are the promoters like in eukaryotes
diverse and complex; regulatory elements can be several kilo base pairs in length from ATG = bending of DNA
what is the termination in prokaryotes
- Rho-dependent
2. Rho-independent
what is the transcription termination in eukaryotes
not known but two models proposed
- torpedo model
- allosteric model
what is the torpedo model
Protein factors recognise terminator signals associated with RNA polymerase II triggering termination process
what is the allosteric model
Poly A signals transcribed to mRNA and two proteins transfer Carboxyl terminal domain of RNA polymerase II to polymerase A signal
how is mRNA made in eukaryotes
- mRNA capped (methyl guanosine at 5’ end)
- mRNA tailed (polyadenosine at 3’ end, usually 200nt of As)
- mRNA spliced (intron boundaries are cut and rejoined to remove intron sequences from the nascent mRNA). Eukaryotic nuclear genes, this is carried out by spliceosome. Prokaryotes, introns are much rarer and of different type
- mature mRNA is transported from nucleus to cytoplasm, where translation will over
what is the Shine-Delgarno sequence
AGGAGGU - AUG (3-10 nucleotides)
prokaryotes, consensus sequence in mRNA
to which ribosomes bind
what makes up a mRNA cap and where is it
Cap is an extra guanosine triphosphate joined backwards (5’ end to 5’ end) onto front of mRNA
what is the capping structure like in prokaryotic mRNA
very few prokaryotic mRNAs, mainly from viruses that infect eukaryotic cells, have specific site in 3’ UTR of mRNA, which ribosome can bind
Most do not, have a cap structure. One subunit of ribosome (plus other factors) binds to cap and move along mRNA looking for first AUG
what are the nucleotides like in the original mRNA
Nucleotide and first couple of nucleotides in the original mRNA have methyl groups attached to them
how does tailing of mRNA in type II genes occur
effect
no terminator of transcription
- transcription continues beyond end of gene. mRNA is cut and string of couple of hundred adenosines (poly A tail) is added
- ribosome can’t bind to cap until whole set of proteins (initiation factors) have bound there
what is molecules are capped and tailed
only mRNA
what is the role of cap and tail
help ribosomes bind
what occurs if the mRNA cap and tail are removed
removed won’t translate efficiently, or translate at all
what does the capping and tailing protect
mRNA from nucleases
what determines the end of tRNA and rRNA
specific terminator sequences
what are genes transcribed by that don’t make functional proteins and why
genes transcribed by RNA polymerase I and those translated by RNA polymerase III do not encode proteins so don’t need to be translated – don’t have these structures or mechanism of termination
examples of eukaryote proteins additional processing
and why
usually addition of molecules to them in order to make them functional, e.g.
- Phosphorylation (addition of phosphate)
- Methylation (addition of methyl groups)
- Glycosylation (addition of carbohydrate chains)
how can proteins be activated
by post-translational proteolysis (trimming of N or C termini or internal cleavage to yield one, or more smaller polypeptides)
how may eukaryote proteins be targeted intracellularly
- out of cell
- to the endomembrane system (vacuole, protein bodies or lysosomes)
- to other organelles (mitochondria and plastids)
why aren’t proteins intracellularly targeted the same in prokaryotes as eukaryotes
prokaryotes don’t have most of these cellular compartments
