Chapter 17 - Expression of Genes Flashcards
functional RNA
tRNA, rRNA
central dogma
explanation of the flow of genetic material:
DNA => RNA => protein
transcription & translation
Beadle and Tatum’s experiment
- bombarded red bread mold with X-rays
- analyzed relationship between metabolic pathway of arginine biosynthesis and mutated genes
-explained the “one gene one enzyme” hypothesis
degeneracy/redundancy of codons
many-to-one structure-function relationship
e.g. more than one codons (synonymous codons) code for a particular amino acid
Sense strand
(non transcribing strand)
-DNA strand that is not used for mRNA transcription
- directed in 5’-3’ direction
- contains same sequence as mRNA (except T)
Antisense strand
(template strand)
-template used for transcription to make mRNA
-antisense because its direction is 3’-5’ as the mRNA needs to be transcribed in 5’-3’ direction
Start codon
AUG - methionine
Stop codon
UAA / UAG /UGA
Site of transcription
eukaryotes - in the nucleus
prokaryotes - in the cytoplasm (nucleoid region)
RNA polymerase (II)
- enzyme for transcription
- binds to promoter region on DNA
- acts like helicase: unwinds DNA double helix
- acts like DNA polymerase III: adds RNA nucleotides to 3’ OH end of transcript
Transcription factors
mediate binding of RNA polymerase
+
initiation of transcription
1) general TF - responsible for all transcriptions in every cell
2) specific TF - works specifically at specific time in a specific cell
Stages of transcription
1) Initiation
- formation of transcription initiation complex
a) RNA polymerase (II) on promotor
b) general, specific transcription factors
c) transcription unit - DNA region downstream of promotor
2) Elongation
- RNA polym II moves along template strand and adds complementary RNA nucleotides to 3’ OH end of transcript
3) Termination
a) in prokaryotes: terminator sequence transcribed, RNA polymerase detaches from DNA template
b) in eukaryotes: polyadenylation signal recognized by nuclease which cleave mRNA from RNA polymerase II
RNA processing in eukaryotes
1) 5’ capping
- adding modified guanine to 5’ end of pre-mRNA
2) 3’ poly A tail
- adding 50-250 adenine nucleotides to 3’ OH end of pre-mRNA
3) RNA splicing
last step of RNA processing
Intron
INTervening RegiON
-non-coding segments
- removed during RNA splicing
- increases probability of crossing over
Exon
Expressed regiON
-coding segments
5’ capping
method of RNA processing in eukaryotes
-adding modified guanine to 5’ end of pre-mRNA
-role: protect mRNA from nuclease, promote export of mRNA through nuclear pore
3’ poly A tail
method of RNA processing in eukaryotes
-adding 50-250 adenine nucleotides to 3’ OH end of pre-mRNA
-role: protect mRNA from nuclease, promote export of mRNA through nuclear pore
RNA splicing
last step of RNA processing in eukaryotes
(occurs in spliceosome - complex of protein & DNA)
- ribozyme removes introns in pre-mRNA
- keeps axons
Alternative RNA splicing
- 2+ final mRNA produced from single mRNA
- depends on which segments are treated as exon/intron
-increase genetic diversity
Large scale mutation
1) alteration in chromosome number
a) aneuploidy (2n+1, 2n-1)
b) polyploidy (3n, 4n etc.)
2) Alteration in structure of chromosome
a) deletion
b) duplication
c) inversion
d) translocation - fragments of one chromosome exchanged with non-homologous chromosome
Small scale mutation
point mutation (changes in one/few nucleotide pairs) 1) nucleotide pair substitution (during DNA rep and DNA repair)
a) silent - no effect (change to synonymous codon)
b) missense - original codon changed to different codon, different amino acid formed
e.g. wild type beta- globin (glutamine) to sickle cell beta globin (valine)
c) nonsense - original codon changed to stop codon
(UAA, UAG, UGA), produced premature polypeptide
2) nucleotide pair insertion/deletion
- causes frameshift
tRNA
- functional RNA
- contains anticodon
- transfer amino acid from cytoplasmic pool to ribosome
- clover shaped (2D), L shaped (3D)
- approximately 20 different tRNAs
1) empty (uncharged) tRNA - no amino acid on 3’ end
2) charged (aminoacyl) tRNA - amino acid attached to 3’ end
3) peptide tRNA - many amino acids attached to 3’ end
aminoacyl tRNA synthetase
covalently bonds tRNA with corresponding amino acid
-amino acid and uncharged tRNA enters active site
-forms aminoacyl (charged) tRNA
ribosome for translation
-composed of rRNA and protein
a) large subunit
- EPA sites
b) small subunit
Translation process
1) initiation
- forms translation initiation complex (mature mRNA, initiator tRNA (methionine), ribosome)
- small ribosomal subunit binds to mRNA
- large ribosomal subunit binds
- hydrolysis of GTP provides energy for assembly
- initiator tRNA bonds to start codon
2) elongation
a) codon recognition
b) peptide bond formation between amino acids
c) translocation of ribosome
3) termination
- occurs when stop codon (UAA, UAG, UGA) reaches A site of ribosome
- releasing factor binds to A site and promotes hydrolysis of initiation complex
Role of GTP during translation
hydrolysis of GTP provides energy for assembly of initiation complex
(mature mRNA, initiator tRNA (methionine), ribosome)
polyribosome
simultaneous translation - where several ribosome are attached to single mRNA
When does gene expression occur?
Usually in the G1 and G2 phase of the cell cycle
Triplet code
four nucleotides (ATGC) 4^3 =64 possible codes to code for amino acids
Where does translation occur?
In both bound and free ribosomes
Central dogma location
Transcription - nucleus
RNA processing - nucleus
Translation - ribosome (free and bound) in cytoplasm
Exceptions to universality of codon table
Mitochondrial and chloroplast DNA - use their own codon table
-explained by endosymbiotic theory
Wobble theory
61 (64 - 3 stop codons) make 20 different amino acids
- explained by redundancy of genetic codon
- flexible base pairing for 3rd codon of mRNA
enzyme required for transcription
RNA polymerase (II)
TATA box
Part of promoter, initiates transcription
Transcription factors
-help the binding of RNA polymerase II to promoter
1) General transcription factor
- responsible for all transcriptions in all cells
2) Specific transcription factor
- works specifically at specific time and in specific cell
tRNA shape
2D - clover shaped
3D - L-shaped
Aneuploidy
Abnormal number of particular chromosome (2n+1, 2n-1)
Polyploidy
Abnormal extra set of chromosome
(3n, 6n)
Alteration in structure of chromosome
1) deletion - removal of fragment
2) duplication - segments existing more than once on same chromosome
3) inversion
4) translocation - exchange of fragments
Point mutation
1) nucleotide pair substitution
- replacement of one nucleotide and its partner with another pair of nucleotides
- during DNA replication and DNA repair
a) silent mutation - change codon to synonymous codon
b) missense mutation - change codon to different codon
c) nonsense mutation - change to stop codons
2) nucleotide pair insertion/deletion
- alteration of reading frame
silent mutation
type of nucleotide pair substitution (point mutation)
- change of codon to synonymous codon
- due to wobble theory, no changes in phenotype
missense mutation
type of nucleotide pair substitution (point mutation)
- change codon to different codon
- formation of different amino acid
e. g. sickle cell anemia (change glutamine to valine)
nonsense mutation
type of nucleotide pair substitution (point mutation)
- change codon to stop codon
- production of premature polypeptide
