DNA Transcription & Translation Flashcards
Ribosome
A ribosome is an organelle composed of ribosomal RNA and ribosomal proteins (known as a Ribonucleoprotein or RNP)
- RNA protein
- Free ribosomes
- Attached ribosomes
Peptide Bond
Polypeptides
AA Structure
Proteins
- Shape is essential to function – Denatured protein no longer functional
- One or more polypeptides
- Composed of amino acids
- 20 amino acids
- Directed by sequence of bases (codons)
Protein Synthesis:
Who decides what proteins must be made?
Who determines the sequence of Amino Acids?
Ribosome
T-RNA
M-RNA
DNA backbone & bonds
OH end: 3’ end
P end: 5’ end
Number is d/t sugar molecule
DNA makeup
- Pentose sugar (deoxyribose)
- Phosphate molecule
- Four nitrogenous bases
- Pyrimidines: cytosine and thymine
- Purines: adenine and guanine
each sugar attaches to a nucleic acid
Pairing of nitrogenous bases
- T and A always bind to one another – through 2 bonds
- C & G bind, through 3 bonds = harder to break
- It is these bonds that hold the structure together
Pyrimidines
cytosine, thymine
(uracil in RNA)
purines
adenine, guanine
Image: DNA molecule w/ sugars, bases, phosphate groups
Which end of the DNA strand can you elongate?
The 3’ end
OH + PPP –> release 2P & H20 –> phospate bond
structure: 3’ vs 5’ end
5’ end has the 5th carbon
3’ end has the 3rd carbon
antiparallel
Basic goal of translation
DNA is a double helix where genes/codons are.
To express the genes as Protein A/B/C… they must be translated
Transcription vs translation
Transcription = DNA → RNA Translation = RNA → protein
What happens after transcription?
In transcription, everything is copied, but not all is necessary
Thus, must next undergo splicing (posttranscriptional)
Introns are filler, they’re spliced out.
Exons are the desired code - they remain and are later translated to protein
Could happen anywhere between transcription & protein synthesis
Chromosomal DNA –> Primary RNA transcript/nuclear RNA –> mRNA
Intronic Character of DNA
Since less than 2% of the human genome encodes proteins and most genes primarily consist of introns, it is difficult to “find” the genes in the DNA sequence
Real genome is buried.
True or False: RNA polymerasemakes an exact mirror image of the strand
False.
takes bases & makes mirror image of strand, in every way reflects original pairing EXCEPT uses uracil instead of thymine.
Transcription Factors
Read and interpret the genetic blueprint thereby controlling transcription or the flow of genetic information from DNA to mRNA
- Regulate timing of transcription
- Regulate tissues in which genes are actively prescribed (e.g., clotting factor VIII primarily in hepatocytes)
- can activate or repress expression of genes
- e.g., protens that function as receptors, enzymes, biomarkers
- Environmental stimuli: hormones involve signaling cascades that can involve transcription factors
- can alter gene expression to promote pathophysiology
Transcription factors image
What opens the DNA helix for transcription?
- RNA polymerase
- Some 50 different protein transcription factors
What is a promoter site?
a sequence of DNA that specifies the beginning of a gene
Transcription Story
- some 50 different protein transcription factors, bound to DNA sequences called transcription factor binding sites/promoter site (usually on 5’ side)
- RNA polymerase binds to the promoter site where TFs are
- Together, RNA polymerase & transcription factors open the DNA double helix - and the TFs position the RNA polymerase between the strands
- RNA polymerase leaves the TFs behind, proceeds in the 3’ to 5’ direction (can be initiated by enhancers - nearby DNA sequences)
- In eukaryotes, 3’ to 5’ movement requires that nucleosomes in front of the advancing RNA polymerase be removed (= unzipped?)
- As RNA polymerase travels along the DNA strand, it is reading ONE strand and assembles corresponding ribonucleotides into a strand of RNA
- Each ribonucleotide is inserted into the growing RNA strand following the rules of base parings (A-U, C-G) - the pyrimidine **U replaces T (U from uridine triphosphate)
- Synthesis of the RNA proceeds in the 5’ to 3’ direction
- As each nucleoside triphosphate is brought in to add to the 3’ end of the growing strand, the two terminal phosphates are removed.
- When transcription is complete, the transcript is released from the polymerase and shortly thereafter, the polymerase is released from the DNA
then to posttranscriptional splicing
Start Code
AUG: tells to read upstream to 3’ end
Template strand, coding strand, and transcription bubble
Template strand: 3’ to 5’ (being read)
Coding Strand: 5’ to 3’ (what mRNA will match, except U)
Transcription bubble: whole unzipped area
Translation
Process by which RNA directs the synthesis of a polypeptide
Site of protein synthesis
Ribosome (on rER)
ribosome moves along the mRNA sequence to translate the AA sequence
Role of tRNA in protein synthesis
tRNA contains a sequence of nucleotides (anti-codon) complementary to the triad of nucleotides on the mRNA strand (codon)
Translation story
just before: mRNA leaves nucleus and travels to ribosome, undergoing postranscriptional modification along the way
- Ribosome binds to mRNA at a specific area
- Ribosome reads mRNA codons and “calls” matching tRNA anticodon sequences
- Each time a new tRNA comes into the ribosome, the AA that it was carrying gets added to the elongating polypeptide chain and the tRNA leaves, mRNA moves long sequence and “calls” a new one
- The ribosome continues until it hits a stop sequence, then it releases the polypeptide and the mRNA
- The polypeptide heads where it needs to go, forms into its native shape and starts acting as a functional protein in the cell (tertiary or quaternary structure essential to correct functioning)
tRNA structure
Site for amino acid attachment on 3’ end
anticodon on loop (3 loops, only one anticodon)
Loop is like a hairpin, has anticodon
How many nucleotides for one amino acid
3 - a codon
How many possible codons
64
16 possible couplets (4x4); codon is series of 3 (4x4x4 = 64)
Codons & amino acids
Can have a number of codes for one amino acid. This is protective - if there’s a mistake, more likely to end up with the correct protein
Problem: some are very polarized, others not. e.g., Valine has no polarity, Thr does - so if you substitute one for another, a structure is polarized that was not meant to be –> shape is changed –> pathophysiologic basis of disease
Start & Stop Codons
Start: AUG
Stop: UAA, UAG, UGA
Protein composition
one or more polypeptides
polypeptides composed of sequences of amino acids
How many termination/nonsense codons?
3
How many codons specify AAs?
61
there are only 20 AAs, thus the genetic code is redundant
universality of genetic code
w/exception of ciliated protozoa and some plants, all organisms use same DNA codes to specify proteins
EXCEPT: in mitochondria - own DNA w/different codes
