Transcription and Translation Flashcards
G0
Non-growing phase of the cell cycle, most cells spend the majority of their lives
Cell is not exerting energy in self-replication and is serving various functions within the body
Transcription
Process by which RNA is manufactured from a DNA template
RNA transcript is created, which copies information in DNA
- rRNA, tRNA, small nuclear RNA (smRNA), and mRNA
Translation
Takes nucleotides sequence of RNA transcript and translates it into the language of amino acids which are then strung together to form a functional protein
Form of regulation of gene expression
Stages of Transcription
Initiation
Elongation
Termination
Transcription Initiation
Group of binding proteins called transcription factors identifies a promoter on the DNA strand assembling into a transcription initiation complex which includes RNA polymerase
RNA polymerase unzips the DNA helix, creating the transcription bubble
Promoter
Sequence of DNA nucleotides that designates a beginning point for transcription
Regulate where on the genome transcription can take place and how often certain sequences are transcribed
Has sequence variability which serves as a regulatory function for tighter or looser bonding and less frequent transcription
Consensus Sequence
Most commonly found promoter nucleotide sequence recognized by a given species of RNA polymerase
Variation from the sequence causes less frequent transcription
Transcription Elongation
RNA polymerase transcribes only one strand of the DNA nucleotide sequence into a complementary RNA nucleotide sequence
Transcribed strand: template strand (antisense strand, -)
Other strand: coding strand (sense strand +) protects partner against degradation
RNA polym moves over DNA in 3’ to 5’ direction
Proofreading in Transcription
No proofreading process, errors are not mutations, and errors are not transmitted to progeny
Most genes are transcribed many times in a cell’s lifetime, so not generally harmful
Transcription Termination
Occurs when a specific sequence of nucleotides known as the termination sequence is reached
Coding Strand in Transcription
Strand that matches the base pair sequence of the newly synthesized RNA strand except for the replacement of Thymine with Uracil.
Also known as (+) sense strand
Template Strand in Transcription
Strand that is transcribed. The resulting RNA strand has the complementary base pair pattern to this strand.
RNA polymerase moves along this strand placing the base pairs that match up with the base pairs on the template strand
AKA (-) antisense strand
When does transcription termination occur?
When a specific sequence of nucleotides are reached, called the termination sequence
Rho proteins can also help to dissociate RNA polymerase from the DNA template strand
What is the main level of activation or deactivation of genes?
Transcription
Regulation of gene expression occurs at level of transcription via proteins called activators and repressors
Often allosterically regulated by small molecules such as cAMP
Enhancers
Short, non-coding regions of DNA found in eukaryotes that function similarly to activators
Act at a much greater distance from the promoter
How is transcription regulation different in prokaryotes?
Primary purpose of gene expression in prokaryotes is to respond to changes in environment
Primary function of gene expression in multicellular organisms is homeostasis; stable unchanging state of intracellular and extracellular comparments
Polycistronic
Several genes encoded in a single transcript
Prokaryotic mRNA
Monocistronic
One gene per transcript
Eukaryotic mRNA
Jacob-Monod Model
Model of prokaryotic genetic regulation in which an operon consists of the operator, promoter, and genes that contribute to a single prokaryotic mRNA
E.g. lac operon
Lac Operon
Bacterial genetic regulation genetic unit that is present in E. coli
Transcribes genes to utilize lactose if glucose is scarce (causing cAMP to bind to catabolite activator protein (CAP) which binds to promoter and lactose is present in sufficient quantities and binds to lac repressor, making it unable to block transcription of lac genes
Gene Repression
Enzyme or protein can be a repressor, which binds to DNA and blocks transcription of a particular gene into mRNA by blocking the formation of the initiation complex
Positive Control
Activation of a promoter, in which the presence of a particular metabolite causes a promoter to enhance the transcription of a gene (maybe cause more stable binding and formation of initiation complex)
Why is the amount of mRNA of a gene in a cell likely related to the amount of protein produced from that gene?
The mRNA gets directly translated to the protein. The mRNA is usually degraded shortly after being transcribed in the cytosol
Therefore, an mRNA that is present in a cell will usually correspond to a protein that has been recently translated from that mRNA
However, many proteins can be translated from one mRNA
Post-Transcriptional Modification
One of the major means through which gene expression is regulated
Both Eukaryotic and Prokaryotic cells
Eukaryotic: all RNA undergoes post-transcriptional processing in nucleus
Prokaryotes: only rRNA and tRNA go through post-transcriptional modification
Primary Transcript
Initial mRNA after transcription in Eukaryotes
AKA pre-mRNA or heterogeneous nuclear RNA (hrRNA)
What is the purpose of post-transcriptional modification in Eukaryotes?
- Helping molecules that initiate translation recognize mRNA
- Protect mRNA from degradation
- Eliminate extraneous sequences of nucleotides from transcript before translation
- Provide a mechanism for variability in protein products produced from a single transcript
What are the ways Eukaryotic mRNA is post-transcriptionally modified?
- 5’ cap, serves as attachment site in protein synthesis and protection against degradation by exonucleases
- Poly-A tail added to 3’ end, aids in export of RNA and in translation initiation
- Splicing out introns from exons
- Alternative splicing creates varying protein products
Why can the human genome produce > 100,000 proteins from 20,000-25,000 protein-coding gene regions?
Alternative splicing: removing and keeping different exons and introns
Use of alternative promoter sites
Terminating transcription at different sites
Different polypeptides can be created from same sequences
Introns
Portions of primary transcript that are removed prior to translation
Generally much longer than exons
Sequences with introns allow for greater alternative splicing and therefore show greater amplification of gene expression
Exons
Portions of primary transcript that become part of mature mRNA and will code for proteins
snRNPs
Small nuclear ribonucleoproteins- contain assortment of proteins and snRNA
Acts as a ribozyme- RNA molecule capable of catalyzing specific chemical reaction
What is the process of Splicing?
Occurs when snRNPs recognize nucleotide sequences at ends of introns. They then pull the introns together, forming an intron loop (lariat)
Then, the introns are excised, and exons are joined together
Splicesome
Complex formed from association of snRNPs and additional associate proteins during splicing
Explain why spatial separation of transcription and translation in Eukaryotes is important.
Spatial separation is a from of genetic regulation. Transcription occurs in the nucleus and primary transcript can be modified before it leaves the nucleus prior to translation.
Prokaryotes have no such separation, and carry out transcription and translation concurrently, meaning post-transcriptional modification is rare
Translation
Process through which a cell creates the protein products that are necessary to carry out the processes of life from mRNA
Translation of nucleotide sequence of mRNA into amino acid sequence of corresponding protein
Triplet Code
Series of three nucleotides code for each amino acid (20 amino acids)
Number of possible combinations of any three nucleotides gives 4^3 = 64
Degeneracy in the genetic code
More than one series of three nucleotides may code for the same amino acid
How is the genetic code Unambiguous?
Any single series of three nucleotides will code for one and only one amino acid
How is the triplet code highly conserved?
Nearly every living organism uses these same codes to translate mRNA sequences into strings of amino acids
Codon
Three consecutive nucleotides on a strand of mRNA comprise a codon
61 codons code for amino acids
Stop codons
AKA termination codons
Three codons which signal an end to protein synthesis; UAA, UGA, UAG
Start codon
AKA initiation codon
Indicates where translation will begin
AUG
Codes for amino acid methionine- always the first amino acid incorporated into a polypeptide during protein synthesis
tRNA
transfer RNA- matches up the triplet code of mRNA with a specific amino acid
Two distinct ends: anticodon loop, and acceptor end
Wobble Pairing
Third base pair in codon can have a little flexibility, explains why multiple codons can code for the same amino acid
Anticodon
Series of three nucleotides which will bind to the complementary codon sequence on mRNA
Ribosome
Specialized translation organelle which can be free-floating in cytosol or attached to rough ER
Small subunit and large subunit, made up of rRNA
- catalytic component of ribosome
- many proteins contribute structural support
Sedimentation coefficient
Measurement of mass, shape, and density for comparing ribosomes
Measured in Svedberg units (S)
Prokaryotic ribosomes: 30S and 50S subunits with combined as 70S
Eukaryotic ribosomes: 40S and 60S with combined 80S
Nucleolus
Organelle that manufactures ribosomes in the nucleus in Eukaryotic cellst
Translation initiation factors
Co-factor proteins
Help 5’ end of mRNA attach to small subunit of ribosome
Translation Initiation
mRNA leaves nucleus through nuclear pores, enters cytosol, and 5’ end attaches to small subunit of ribosome
tRNA with 5’-CAU-3’ anticodon sequesters amino acid methionine and settles into P site (peptidyl site) of ribosome
Large ribosome subunit joins and forms initiation complex
When does the most regulation of translation occur?
During initiation, through recognition or lack of recognition between ribosomal subunits and secondary structure of mRNA transcript
Translation Elongation
Ribosome slides down mRNA strand one codon at a time in 5’ to 3’ direction while matching each codon to complementary tRNA anticodon.
Amino acids enter in A site, form bond with amino acid in P site, and AA in A site slides to P site while AA in P site slides to E site where the tRNA leaves
Translation Termination
Ribosome reaches a stop codon (nonsense codon) at mRNA in A site, release factors bind to A site, allowing water to add to end of polypeptide chain, which then is freed from tRNA and ribosome
Ribosome subunits then break apart
Chaperones
Proteins that assist in folding conformation of proteins
Post-translational modification
Regulation of gene expression by affecting which products
May add sugars, lipids, or phosphate groups to proteins
Polypeptide can be cleaved or separate polypeptides may join together
What determines the final destination of a protein?
Where it is translated
Proteins translated on free-floating ribosomes in cytosol function in cytosol
Proteins translated on ER-bound ribosome get injected into ER lumen to become: membrane bound proteins of nuclear envelope, ER, Golgi, lysosomes, plasma membrane, or to be secreted from cell
Signal Peptide
20 amino acid sequence near the front of the polypeptide that signals a protein to go to ER
Recognized by protein-RNA signal recognition particle (SRP) which carries entire ribosome to receptor protein on ER
Signal peptide is usually removed by enzyme
May also be attached to polypeptides to target them to mitochondria, nucleus, etc.
How do amino acids bond together in translation?
Amino acids bond together through rRNA-based peptidyl transferase, which catalyzes dehydration reaction forming a peptide bond
C-terminus of old amino acid attaches to N terminus of new amino acid in A site of ribosome
