Gene Expression And Regualtion In Eukaryotes Flashcards
Central dogma
Flow of information from dna to ran to make a protein
3 stages of the central dogma
Dna replication
Transcription
Translation
Dna replication
-helicases unwind the double helix
-single strand binding proteins stabilise the unwound paternal dna
-The leading strand is synthesised continuously in the 5’-3’ direction by dna polymerase
-the lagging strand is synthesise discontinuously.Primase synthesis a short rna primer which is extended by dna polymerase to form an Okazaki fragment
-after rna timer is replaced by dna, dna ligase joins Okazaki fragment to the growing strand
Okazaki fragment
Sequences of dna nucleotides which are synthesise discontinuously and joined by dna ligase
Okazaki fragment
Sequences of dna nucleotides which are synthesise discontinuously and joined by dna ligase
Transcription
-initiation
-Elongation
-Termination
Done by rna polymerase
Processing of primary mrna into mature MRNA in the nucleus
Translation
-it is the synthesis of proteins on ribosomes in the cytoplasm
Initiation
Elongation
Termination
Key stages where gene expression may be regulated
-chromatin modification
-transcription initiation
-post transcription
-post translation
Regulation of gene expression On a dna level(chromatin)
-genes within heterochroamtin are usually not expressed
-active genes are within euchromatin are usually expressed
-chemical modifications to the histones regulates expression,Histone modification include:
1)histone acetylation:it unwinds dna as acetyl groups are attached go lysine in history tails acetylation adds negative charge ,reduces attraction to dna loosens the structure and permits transcription
2)histone methylation:
- The addition of methyl groups (methylation, CH3) to histone tails can reduce transcription – Transcription repression
-nucleosomes pack tightly together,transcription factors cannot bind dna,genes are not expressed
• No change to charge of the histone protein
• Methylation can attract other chromatin remodelling proteins to either
loosen the structure or block TF binding
• Less dynamic than acetylation but can be reversed
• Mono, di- or tri methylation
-Dna methylation
• Attaching a methyl group (-CH3) to a cytosine nucleotide base within a cytosine- guanosine (CG) sequence (CpG island)
• Methylation of CpG sites in promoter regions, prevents the DNA from being transcribed = Gene silencing
• Roles in cellular differentiation and long term inactivation of genes e.g. expression of maternal or paternal alleles, X chromosome inactivation, imprinting
Chromatin
-it’s a complex of equal parts of DNA and
proteins that forms chromosomes
-The nucleosome (beads) is the fundamental subunit of chromatin
• 8 protein molecules (histones)
• Positively charged AAs bind to negatively charged DNA
-Chromatin could be heterochromatin or euchromatin
Heterochromatin
-deeply stained
-Very comfort/condensed dna
-genes not likely to be expresses
Euchromatin
-Lightly stained
-Less compact
-Loosely packed DNA = accessible!
Regulation of gene expression:transcription
Characteristics involved:
UPSTREAM of gene
• Promoter and Transcription factors
• Proximal control elements
• Enhancer (distal control elements)
DOWNSTREAM of gene
• Poly-A signal sequence
• Transcription termination region
Transcription initiation:
- Activator proteins bind to distal control elements (enhancer)
- A DNA bending protein brings the bound activators close to the promoter
- The activators bind to general transcription factors to form an active transcription initiation complex,
- Promotes RNA Pol binding to the promoter and transcription begins
Regulation of Gene Expression: Post-transcription
- 5’ cap (methylated Guanosine)
- poly-A (adenine) tail at the 3’
primary (pre) mRNA- RNA Splicing: removal of introns (intra-genic regions) by spliceosome
Next step: translocation to the cytoplasm from the nucleus
- RNA Splicing: removal of introns (intra-genic regions) by spliceosome
- Alternative RNA splicing
Variable processing of exons creates a family of proteins / spliced variants
Function of The 5′ cap has in post transcriptional gene expression
- Regulate nuclear export of mRNA;
- Prevent degradation of mRNA by exonucleases;
- Promote translation by facilitating ribosome attachment
- Promote 5′ proximal intron excision/splicing out
Function of the 3’ poly-A tail in post transcriptional gene expression
- promote the export of mRNA from the nucleus
- protect the mRNA from degradation
- facilitate ribosome attachment to mRNA
Other non-coding (nc) RNAs
• rRNA (ribosomal) and tRNA (transfer) are non-coding RNAs – NO
protein!
• Long nc RNA (lncRNA) >200 nucleotides, diverse functions
• Micro RNAs (miRNAs) – small, single stranded RNA molecules, bind to complementary mRNA sequences. Lead to degradation of the target mRNA or block its translation- post transcriptional gene silencing
• Small interfering RNAs (siRNAs)– similar to miRNAs, ~20bps in length, block gene expression via RNA interference (RNAi) – gene silencing
Regulation of Gene Expression: Post-translation
Protein processing and protein degradation
• Proteolytic (protein) processing e.g. pre-pro-hormones
• Chemical modification (introducing functional groups)
e.g. phosphorylation, glycosylation, targeting for transport
• Protein degradation(ubiquitination)
Protein degradation: Ubiquitination
• Ubiquitin is added to proteins destined for degradation
• A marker (76aa) ‘Death tag’
• Recognised by proteosome
• The proteosome, breaks down proteins in to smaller fragments (7-9 aa)
How is Prokaryotic and Eukaryotic Gene Expression
similar?
1)Both require the participation of regulatory proteins known as transcription factors
• prokaryotes: activator and repressor
• eukaryotes: activator proteins act on enhancer
2) In both prokaryotes and eukaryotes, control of gene expression can occur at the level of:
• transcription
• translation
• protein activity (post-translation)
How is Prokaryotic and Eukaryotic Gene Expression different?
Prokaryotes: Mostly On
• Most genes of a bacterium constitutive.
• The majority of products are constantly made
• Repressible systems are more common than inducible systems.
Eukaryotes: Mostly Off
• The vast majority of genes in a mature eukaryotic cell are turned off.
• Specific proteins and environmental conditions are necessary before a gene can • be turned on
How is Prokaryotic and Eukaryotic Gene Expression different?
1)
-Prokaryotes: Mostly On
• Most genes of a bacterium constitutive.
• The majority of products are constantly made
• Repressible systems are more common than inducible systems.
-Eukaryotes: Mostly Off
The vast majority of genes in a mature eukaryotic cell are turned off.
• Specific proteins and environmental conditions are necessary before a gene can
• be turned on
2) Prokaryotes: Naked DNA
Eukaryotes: Nucleosomes
• Eukaryotic DNA is packaged as chromatin: octomer of histones + DNA
3)
Prokaryotes: One RNA polymerase
Eukaryotes: Three RNA polymerases
• The proteome is transcribed by RNA polymerase II. • RNA polymerase I rRNA
• RNA polymerase III tRNA
4)
Prokaryotes: No modification of mRNA transcript
• What you see is what you get.
Eukaryotes: Extensive modification of mRNA transcript • 5’ ends capped
• 3’ Poly A tail
• introns removed and exons spliced
Prokaryotes:
• Transcription and Translation take place simultaneously
Eukaryotes:
• Transcription and RNA processing in the nucleus; translation in cytoplasm
