(non transcribing strand) -DNA strand that is not used for mRNA transcription - directed in 5'-3' direction - contains same sequence as mRNA (except T)

- 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

INTervening RegiON -non-coding segments - removed during RNA splicing - increases probability of crossing over

last step of RNA processing in eukaryotes (occurs in spliceosome - complex of protein & DNA) - ribozyme removes introns in pre-mRNA - keeps axons

Chapter 17 - Expression of Genes Flashcards by SM Premed

functional RNA

tRNA, rRNA

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central dogma

explanation of the flow of genetic material:
DNA => RNA => protein

transcription & translation

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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

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degeneracy/redundancy of codons

many-to-one structure-function relationship
e.g. more than one codons (synonymous codons) code for a particular amino acid

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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)

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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

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Start codon

AUG - methionine

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Stop codon

UAA / UAG /UGA

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Site of transcription

eukaryotes - in the nucleus
prokaryotes - in the cytoplasm (nucleoid region)

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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

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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

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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

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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

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Intron

INTervening RegiON
-non-coding segments

removed during RNA splicing
increases probability of crossing over

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Exon

Expressed regiON
-coding segments

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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

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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

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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