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Chapter 11

BIOB11H3 Chapter Notes - Chapter 11: Riboflavin, Frameshift Mutation, Leucine


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dan Riggs
Chapter
11

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Chapter 11 Gene Expression: From Transcription to Translation
11.1 The Relationship Between Genes and Proteins
- Archibald Garrod 1908: People have rare inherited disease caused by enzymes alcaptonuria (disease b/c urine
becomes dark when exposed to air)
Patients lacked enzymes in blood that oxidizes homogentistic acid compound from breaking down a.a
(ohenlyalanine and tyrosine)
- George Beadle and Edward Tatum 1040: NEUROPORA Bread mould that forms in normal simple medium
(organic C-sour e, inorganic salts and biotin)
Some organisms should be sensitive to
enzymatic deficiencies
Irradiate mold spores and screen them for
mutation tested for ability to grow in
minimal medium
2 cells unable to grow on minimal medium:
1 needed vitamin B6/pyridoxine and other
needed B1/thiamine
Gene carries information for construction
on particular enzyme enzymes
composed of >1 polypeptide chain (each
encoded w/ own gene) = “one gene- one
polypeptide”
- What is the molecular nature of the defect
in a protein cause by a genetic mutation
Mutation causing sickle cell anaemia
Hemoglobin (4 polypeptides)
Cleave preparation in normal and sickle cell
forms of haemoglobins
Analyzed peptide fragments by paper
chromatography
Of 30, 1 migrated differently in 2
preparation substitution of valine
Overview of the Flow of Information through
the Cell
- Messenger mRNA/ - Intermediate b/w gene and
polypeptide
Discovered by Francoise Jacob and Jacque Monod
Assemble d as complementary copy of 1 DNA
strand
- Transcription: Synthesis fo RNA DNA
- Used to allow cell to separate information storage
form information utilization info imparted to
smaller mobile nucleic acid to cytoplasm
In cytoplasm mRNA serves as template to
incorporate a.a to nucleotide sequence
- Also used to greatly amplify synthetic output
- Translation: Protein synthesis in cytoplasm
Requires participation of many components
RIBOSOMES
Ribosomes are nonspecific components of
translation machinery can be programmed to translate information
- Ribosomal RNA/rRNA: Ribosomes with both proteins and RNA, each transcribed from 1 of DNA strands
Provide structural support and catalyze chemic rxn
- Transfer RNA/tRNA: Translate information in mRNA to amino acid “alphabet”

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- RNA gold to complex 3-D shape
RNA folding driven by formation of regions having
complementary ba se pairs
Base-paired regions form double-stranded/helical
“stems” – connected to signle0stranded loops
RNA contain nonstandard base pairs and modified
N-bases
- RNA important in cellular metabolism: small
nuclear RNA/snRNA, small nucleolar
RNA/snoRNA, small interfering RNA/siRNA,
microRNA/miRNA
11.2 An Overview of Transcription
in Both Prokaryotic and
Eukaryotic Cells
- Transcription DNA strand
provides info for synthesis of RNA
strand
- DNA-Dependent RNA
polymerase/RNA Polymerase;
Responsible for transcription in
prokaryotes + eukaryotes
- Template Strand RNA sequence complementary to 1 DNA strand
- RNA Synthesis:
1. Association of polymerase with DNA template
DNA site where RNA polymerase binds to is promoter
Additional proteins called transcription factors Help RNA polymerase
recognize promoter
RNA polymerase move in 31 51 direction DNA unwinds, assembles
complementary RNA strand
RNAn+ NTP RNAn + PPI; NTP = Ribonucleoside trisphosphate substrates
2nd reaction PPI NPI; catalyzed by pyrophosphatase, hydrolyze to inorganic
phosphate to make energy
2. Incorporate complementary nucleotides in NRNA chain, only if able to form
proper base pair
3. When polymerase move past certain part of DNA double helix reforms, RNA
chain doesn’t associated w/ template as DNA-RNA hybrid
4. RNA polymerase needs to remain attached over long stretches of template
(processive) but loose enough so it can move form nucleotide to the next
Transcription in Bacteria
- Bacteria have 1 type of RNA polymerase (5 subunits to make core enzymes)
- RNA polymerase formed by purified polymerase
- If purified accessory polypeptide called sigma factors added to RNA polymerase,
transcription begins at selected locations increases enzymes affinity for
promoter sites
- Enzymes separates to 2 DNA strands
- Strand separation makes template strand
- Once 10-12 nucleotides incorporated, enzymes undergoes
major change in conformation and transformed to
transcriptional elongation complex
- Bacterial promotes at region of DNA before imitation site of
RNA synthesis
- Upstream Portions before initiation site (to 31); Downstream
portions succeed it (to 51)

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- Consensus Sequence: TTGACA SEQUENCE (-35 ELEMENT); most
common version of conserved sequence
- Ring-shaped protein called rho terminates bacterial transcription
Transcription and RNA processing in Eukaryotic Cells
- Robert Roedler 1969: Eukaryotic cells have 3 distinct
transcribing enzymes
- Understanding of transcription in eukaryotes advanced by X-ray
crystallographic structure of Yeast RNA polymerase II
- Difference in prokaryotes and eukaryotes is transcription factors
role in every aspect of transcription
- All major types of eukaryotic RNA are derived from precursor RNA molecules
- Initial precursor RNA is primary transcript/pre-RNA
- Corresponding segment where pre-DNA transcribed is transcription
unit
11.3 Synthesis and Processing of Messenger RNAs
- DNA sequences are repeated 100s of times rDNA clusters in 1-few
regions of genome
- Cluster or rDNA = nucleoli/nucleolus that produce ribosomes
- Nucleolus made of nascent ribosomal subunits
Synthesizing the RNA Precursor
- In amphibian oocytes, amount of rDNA in cell greatly increased like number
of nucleoli provide large # of ribosomes to for egg to begin
embryonic development
- Development of techniques by Oscar Miller Jr.: “Genes in action:
1. Numerous genes for rRNA situated one after another along DNA
molecule = tandem arrangement
2. Each fibril coming from DNA as a branch is nascent rRNA
transcript
3. Nascent RNA transcripts contain associated particles consist of
RNA and protein to covert rRNA rRNA
4. Region of ribosomal gene cluster is not transcribed non-
transcribed space, prestin in repeated genes
Processing the rRNA Precursor
- Have 4 distinct ribosomal RNA:” 3 large (28S 5.8S, 5.S RNA
molecule) and 1 small (18S)
- 28S, 18S and 5.8S are carved by various nucleases from single
primary transcript (pre-rRNA)
- 5S synthesized from separate RNA precursor outside nucleolus
- Pre-rRNA made form e nucleotides and pseudouridine residues
methylated (not sure what these groups do may protect parts
from cleavage, promote folding?)
- Modifications after nucleotides are incorporated to RNA =
posttranscriptional
- Synthesis can be flowed by incubating cells in methioinine
(radioactively labelled) to make most cells a methyl group donor
transferred from methionine to nucleotides in pre-RNA
- Role of snoRNAs: small, nucleolar RNA/snoRNA: packed w/
particular proteins to form particles called snoRNPS (small,
nucleolar ribonucleoproteins)
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