DNA, Genes & Protein Synthesis Flashcards
RNA
Sugar is ribose
Uracil is one of the nitrogen bases
Single stranded
DNA
Sugar is deoxyribose
Thymine is one of the nitrogen bases
Double stranded
Transcription (basics)
Completed in nucleus
Creation of RNA from a DNA template
mRNA (messenger) carries the genetic message to the cytoplasm
why is an RNA intermediate used instead of DNA for protein synthesis?
prevents the degradation of/damage to DNA if it enters cytoplasm
-multiple mRNA molecules can enter cytoplasm to mass produce proteins (ex. antibodies)
Translation (basics)
Completed at a ribosome
Synthesis of a polypeptide (amino acid sequence) from mRNA
Prokaryotes: protein synthesis
Lack of a nucleus changes protein synthesis
Translation of mRNA begins while transcription is happening
Eukaryotes: protein synthesis
Transcription occurs in the nucleus
RNA processing occurs inside the nucleus
Translation is completed outside the nucleus
Genetic Code
20 different amino acids
Three DNA nucleotides code for one amino acid (providing 64 options)
The gene determines the sequence of bases in the mRNA strand, which determines the amino acid
Genetic Code: codons
mRNA triplets that code for one amino acid
Written & read in the 5’ to 3’ direction
Begins with the start (initiation) codon & end with a stop codon
Genetic Code: evolutionary significance
nearly universal! Results from a common ancestor shared by all life Genes from one species can be transcribed and translated from another (in a laboratory of course) medical research
Transcription: initiation
RNA polymerase: enzyme that opens DNA strands
and hooks together RNA nucleotides
Only works in the 5’ to 3’ direction of the RNA strand
being synthesized (read 3’ to 5’ on the template strand)
No primer needed
Transcription factors bind RNA polymerase to the
promoter creating the transcription initiation
complex
The TATA box (on non-coding strand) will help the transcription factors bind to the promoter (located 25 bases upstream of transcription site)
RNA polymerase binds to the promoter then
unwinds & opens the DNA strand to begin RNA
synthesis
transcription (initiation): promoter
nucleotides that determine where transcription begins & RNA polymerase attaches
transcription (initiation): terminator
a nucleotide sequence that signals the end of transcription
transcription (initiation): Transcription unit
the portion of DNA that is transcribed into RNA
role of transcription factors in transcription
-regulate transcription by determining whether or not RNA polymerase attaches to DNA
Transcription: elongation
RNA polymerase moves downstream (5’ to 3’) along
the DNA template exposing nucleotides as it unwinds
RNA nucleotides are added to the 3’ end of the RNA strand
During elongation RNA molecule peels off the DNA
template allowing DNA to reform
Multiple RNA polymerases can follow each other
allowing one gene to be transcribed several times at
once
Transcription: termination (prokaryotes)
transcription continues until a terminator sequence is reached which causes the
RNA polymerase to detach
Transcription: termination (eukaryotes)
the pre-mRNA is cut free while RNA polymerase continues to transcribe
At the polyadenylation signal (AAUAAA) proteins cut
the pre-mRNA
RNA polymerase continues transcribing until it
eventually falls off the DNA (what causes this is still
not known)
mRNA processing: alteration of ends
Pre-mRNA is modified in the nucleus to help with export, prevent degradation of the genes, assist in attachment to the ribosome
5’ cap: modified guanine nucleotide is added- helps mRNA exit through nuclear pore and attach to ribosome
Poly-A tail: 50 to 250 adenine nucleotides are added to the 3’ end- acts as a buffer to prevent mRNA degradation in cytoplasm by enzymes (exonucleases)
-UTR: untranslated region for ribosomal attachment
mRNA processing: RNA splicing
the removal of large sections of previously synthesized RNA Introns: non-coding segments of RNA Exons: parts of the RNA sequence that are expressed Spliceosomes (small proteins & RNAs) are able to recognize the splicing sites & remove introns
mRNA processing: evolutionary significance
Play a role in gene activity
Alternative RNA splicing:
Provides extra chances for genetic diversity through
exon shuffling during cross over
mRNA processing: Alternative RNA splicing:
different polypeptides can
arise from one gene based on which introns and
exons are spliced
translation: tRNA
transfers amino acids from the cytoplasm to the ribosome (uber driver)
Uses anticodon on tRNA to bind to the specific codon & deliver correct amino acid
Reusable
-more tRNA than amino acids because 2 or 4 tRNA molecules will bind to the same amino acid
translation: ribosomes
Made up of two subunits (made of rRNA) that
codons and anticodons
Each ribosome has three binding sites:
P site, A site, E site
translation: ribosomes (binding sites)
P site (peptidyl-tRNA): holds the tRNA carrying the
polypeptide chain
A site (aminoacyl-tRNA): hold the tRNA carrying the
next amino acid
E site (exit site): releases the tRNA
translation: initiation
The small ribosomal unit brings mRNA and an
initiator tRNA (carrying methionine) together at the
start codon
Then the large ribosomal subunit attaches forming
the translation initiation complex
The initiator tNRA is moved to the P site & the next tRNA enters the A site as elongation begins
translation: elongation
Amino acids are added one by one with the help of
elongation factors
Occurs in 3 stages:
Codon recognition:
Peptide bond formation
Translocation
-multiple ribosomes can attach to one mRNA strand, synthesizing multiple polypeptide chains from the same strand
translation: elongation (codon recognition)
the anticodon on the tRNA pairs with the codon on the A site of mRNA
translation: elongation (Peptide bond formation)
the large subunit creates a peptide bond between the new amino acid in the A site and polypeptide chain in the P site
translation: elongation (Translocation)
tRNA is moves from the A site to the P site (this moves the tRNA from the P site to the E site)
mRNA moves long with the tRNA
mutations
changes in the genetic material of a cell/virus
point mutations
chemical changes in just one base pair of gene
substitutions: base pair
replacement of nucleotide and its partners with another pair of nucleotides
substitutions: missense
altered codon still codes for one amino acid but does not make right sense (codes for different amino acid)
substitutions: nonsense
change in codon for amino acid to a stop codon, causes translation to be terminated prematurely
insertions & deletions
additions/losses of nucleotide pairs in a gene
mutagens
physical and chemical agents that interact with DNA in various ways to cause mutations
translation: termination
Elongation continues until the stop codon (mRNA) is
reaches the A site
Release factors binds the stop codon in the A site
which causes the polypeptide chain to by
hydrolyzed & released from the exit tunnel
The translation unit comes apart
