Transcription & Translation (4) Flashcards
gene
linear segment of dna that contains the information needed to synthesize or “express” a particular gene product (rna or protein)
- made up of dna
- codes for rna and proteins
What are the two steps of protein expression?
- transcription of mRNA from DNA
- translation of mRNA to produce a polypeptide chain
Transcription relies on …
complementary base-pairing to copy the nucleotide sequence information present in one strand of the double helix
Messenger RNAs are processed prior to translation …
in eukaaryotic cells this involves the removal or splicing of introns
Where does translation take place?
In ribosomes, and relies upon a highly conserved genetic code
What are mutations?
- changes in the genetic information encoded in dna
- can change the sequence of amino acids in the protein formed from the mutated gene
Central dogma proposed by Francis Crick
DNA (transcription) RNA (translation) Protein
What direction is RNA synthesized in?
5’ to 3’ direction
How is RNA synthesized?
By an RNA polymerase from a DNA template
needs
- nucleoside triphophates (NTP)
- Mg at suitable conditions
Does RNA polymerase need a primer?
No
RNA polymerase works with …
- other proteins called trans-acting factors/transcription factors
Transcription factors recognize …
- cis-acting elements (specific sequences on theDNA)
What do cis-acting elements include and what can they do?
- promoter
- suppresor
- enhancer
they can affect the amount, rate, and accuracy of the transcription
The promoter in eukaryotes affects the …
accuraty and amount of transcription
What the enhancers and suppresors called and what do they do?
- Regulatory regions
- when bound by transcription factors result in more active or less active transcription
What happens when the RNA polymerase and ____ bind to the promoter?
- transcription factors
- open the double helix and start transcribing RNA by incorporating NTPs (C U A G) complimentary to the template RNA strand.
- the opposite strand is not transcribed
Where does transcription stop?
the transcription stop site
What the the transcription stop site denoted by in eukaryotes?
the sequence AATAAA
What does RNA polymerase I transcribe?
ribosomal RNA (rRNA)
What does RNA polymerase II transcribe?
messenger RNA (mRNA)
What does RNA polymerase III transcribe?
transfer RNA (tRNA) and one type of rRNA
Where is eukaryotic RNA processed and translated?
processed in the nucleous, translated in the cytoplasm
Major type of processing 1: Capping & Polyadenylation
Capping happens and at the 3’ end a stretch of poly-As (adenines) is added by an enzyme poly(A) polymerase
Capping process
the phosphate group of a modified form of guanosine triphosphate (GTP) is added to the 5’ end of RNA
What does capping do?
- protects mRNA
- marks the 5’ ends as the starting point for translation
What RNAs are capped and polyadenylated?
only mRNA, to increase their stability
polyA+RNA is …
not found in prokayotes
RNA processing: Splicing
During splicing introns are removed and exons are are joined together
almost all eukaryotic RNA undergo splicing and other processing, except genes that lack introns
Introns:
intervining sequences that do not code for a protein
Exons:
expressed regions coding for a protein or a segment of a protein
What does NTR refer to?
non-translated regions present in processed RNA at 5’ and 3’ ends, that do not code for a protein
Autocatalytic RNA
rna that splice themselves
Splicesome
in some RNA splicing occurs in this complex, made up of small nuclear RNAs, small nuclear ribonucleproteins, and other proteins
George Beadle and Edward Tatum
generated mutations of a fungus so it could not grow on a minimum medium
- one enzyme is coded by one gene
- any mutations is a gene will result in a mutated enzyme
one gene …
one enzyme or one polypeptide
How is information communicated from DNA to protein
- dna is first transcribed into mRNA which is translated into proteins using adapters (tRNA) using a coding system (codons)
Why are codons used?
Codons/coding systems are used because DNA and RNA only have 4 bases, but protein has 20 amino acids
___ bases code for ___ amino acid(s)
3 bases code for 1 amino acid, so 64 amino acids are possible
Start codon (starts polypeptides)
AUG
Stop codons
UGA, UAG, UAA
degeneracy
multiplicity or redundancy of codons coding for one amino acid
- some amino acids were coded by more than one codon
What are the three types of mutations?
- point mutations
- insertions or deletions
- inversion or translocation
Point mutations
- single base paire is changed or substituded with another
- if this mutation results in a different amino acid it is called a missense mutation
- if the change resulted in in the protein sequence stopping it is called a nonsense mutation
Insertions or Deletions
- one or more bases are inserted or deleted resulting in a frameshift mutations
- can happen at the chromosomal level
Inversion and Translocation
- two segments of a chromosome can be inverted or translocated
Mutagenesis; what can cause mutations?
- natural
- chemical mutagens
- physical mutagens
Where does protein synthesis occur in eukaryotic cells?
in the cytoplasm
Transfer RNA
- acts as an adapter between mRNA and ribosomes during protein synthesis
- different types specific to the amino acids
- amino acids are activated and attached at the 3’ region
- have an anticodon complementary to the codon in mRNA
Inosine:
sometimes the third base in tRNA, can recognize A, U, or C on mRNA
modified form of Guanine
Wobble
flexibility of the third base to bond with more than one codon
rRNA
- major component of ribosomes
- most abundant RNA
- couple the mRNA and tRNA carrying various amino acids
- make peptide bonds between amino acids to make proteins
mRNA
- carries message from the gene to be translated into protein
- has a poly(A) tail and travels cytoplasm to be translated
- processed mRNA carry regions that are recognized by ribosomes to start to protein synthesis
- translation starts with the AUG codon
Amino Acid Activation
- ## amino acids must be activated and attached to the 3’-OH group of tRNA to transfer to the elongating polypeptide
Enzymes in Amino Acid Activation
- aminoacyl-tRNA synthetases
- binds to right amino acid at its -COOH terminus and attached an AMP by hydrolyzing ATP
- the enzyme recognizes the tRNA by its secondary structure and anticodon
- it then transfers the amino acid to the 3-OH group of the correct tRNA and the AMP is detached. then it releases the aminoacyl-tRNA
What are the three stages of protein synthesis?
Initiation, elongation, and termination
Initiation
- subunit of ribosomes recognize the ribosome binding site at the 5’ NTR
- special tRNA binds to the P-site
- 3 initiation factors (proteins) and GTP bind to form initiation complex
- large subunit joins with the small subunit and initiation complex
- GTP is hydrolyzed in this step
What are the three steps of elongation?
- Codon recognition
- Peptide bond formation
- Translocation
Codon recognition
Elongation factor [EF (proteins)] brings aminoacyl tRNA and places it on the A site. GTA attached to EF is hydrolyzed
Peptide bond formation
- peptidyl transferase activity of ribosomes transfers methionine (growing polypeptide) from P-site tRNA to A-site and makes a peptide bond between the carboxyl terminus of previous amino acid and amino terminus of incoming amino acid
- tRNA at P-site moves from P-site to E-site (exit) and leaves ribosome
Translocation
-tRNA at A-site translocated to to P-site, another GTA is hydrolyzed
- mRNA codons and corresponding tRNA are H-bonded
- now the next codon moves to the A-site
Termination
- polypeptide elongation continues until the stop codon on mRNA reaches the A-site
- release factor protein binds to stop codon and makes peptidyl transfease add H2O to growing polypeptide chain
- releases completed polypeptide from tRNA at P-site and from the ribosome
- ribosome subunits separate and go on to start another polypeptide synthesis
Polysome
one mRNA can be translated simultaneously by several ribosomes making several polypeptides from one template
What are the examples of protein processing?
- zymogen activation
- disulfide bonds
- hydroxylation
- glycosylation and myristylation
Zymogen activation
inactive protein that becomes active after processing
Disulfide bonds
formed between two cysteines
Hydroxylation
some amino acid chains are chemically modified
Glycosylation and myristylation
.. of some residuals on the protein occur in the golgi apparatus
