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BIO130 Section One Guide (6A)

8 Pages
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Department
Biology
Course Code
BIO130H1
Professor
Kenneth Yip

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Chapter 6 How Cells Read the Genome: From DNA to
Protein
-Proteins working closely with one another often have their genes located on different
chromosomes, and adjacent genes often encode proteins that have little to do with
each other
-DNA uses RNA as in intermediary to direct protein synthesis
oWhen the cell needs a particular protein, the nucleotide sequence required is
first copied into RNA (transcription ) and then these RNA segments are
used directly as templates to direct the synthesis of the protein (translation )
oDNA RNA protein
From DNA to RNA
Portions of DNA Sequence Are Transcribed into RNA
-Step 1 is to copy a gene into an RNA nucleotide sequence (transcription )
-RNA is a linear polymer like DNA made of A, U (not T), G, C nucleotides linked
together by phosphodiester bonds
oDiffer from DNA in that the nucleotides are ribonucleotides; they contain
the sugar ribose rather than deoxyribose
-RNA is typically single-stranded, but may fold onto itself to have double-stranded
sections
Transcription Produces RNA Complementary to One Strand of DNA
-Transcription begins with the opening and unwinding of a small portion of the DNA
double helix
-One of the two strands acts as the template for the synthesis of an RNA molecule
oNucleotide sequence of the RNA chain is determined by complementary base-
pairing between incoming nucleotides and the DNA template
oIncoming ribonucleotides are covalently linked to the growing RNA chain
-**unlike DNA replication, RNA does not remain hydrogen-bonded to the DNA
template strand
oInstead, as the RNA chain forms, the DNA helix reforms
-RNA molecules are considerably shorter than DNA molecules
-RNA polymerases enzymes that perform transcription
oThey catalyze the formation of phosphodiester bonds linking nucleotides
together
oUnwinds DNA just ahead of the active site for polymerization
oIts substrates are ATP, CTP, UTP, and GTP
- Many RNA copies can be made of a single gene on a DNA strand
-Differences between RNA and DNA polymerase:
www.notesolution.com
oRNA polymerase catalyzes the linkages of ribonucleotides, not
deoxyribonucleotides
oRNA polymerase can start an RNA chain without a primer (accuracy not
required)
-Although RNA polymerases are not as accurate as DNA polymerases, when an
incorrect ribonucleotide is added, the polymerase can back up, perform an excision
reaction using water; releasing nucleoside monophosphate (reverse polymerization)
-**template-dependent nucleotide polymerizing enzymes seem to have arisen
independently twice during the early evolution of cells (RNA/DNA polymerase are
seemingly unrelated)
Cells Produce Several Types of RNA
-mRNA (messenger RNA) RNA molecules that are copied from genes that specify
the amino acid sequence of proteins (found on DNA) Comprises 3-5% of total RNA
-rRNA (ribosomal RNA) - form the core of ribosomes majority of RNA cells
-tRNA (transfer RNA) form the adaptors that select amino acids and hold them in
place on a ribosome for incorporation into protein
-snRNA (small nuclear RNA) direct the splicing of pre-mRNA to form mRNA
-snoRNA (small nucleolar RNA) used to process and chemically modify rRNAs
-miRNA (microRNA), siRNA (small interfering RNA) key regulators of
eukaryotic gene expression
Signals Encoded in DNA Tell RNA Polymerase Where to Start and Stop
-the initiation of transcription determines what protein will be produced and at what
rate
-the bacterial RNA polymerase core enzyme synthesizes RNA using a DNA template
-a detachable subunit sigma () factor associates with the core enzyme and helps
with reading DNAs signals that tell it where to begin transcribing
otogether, factor and core enzyme are known as the RNA polymerase
holoenzyme
othis complex weakly adheres to bacterial DNA when the two collide, and the
holoenzyme slides down the DNA
when the holoenzyme slides into a region of the DNA double helix
called a promoter, the polymerase binds tightly to this DNA
-promoter special sequence of nucleotides indicating the starting point for RNA
synthesis
-through the factor, the holoenzyme recognizes the promoter by making specific
contacts with the portions of the bases exposed on the outside of the helix
www.notesolution.com
-when the RNA polymerase holoenzyme binds to the promoter, the double helix is
opened to expose some nucleotides does not require ATP hydrolysis energy
othe polymerase and DNA undergo reversible structure changes that result in
a state more energetically favourable than that of the initial binding
-after the first 10 nucleotides have been added to initiate the RNA chain, the core
enzyme breaks its interactions with the promoter, weakens interactions with the
factor, and moves down the DNA synthesizing RNA
-the enzyme finally stops when it reaches a second signal in the DNA, the
terminator, where the polymerase stops and releases both DNA and RNA
-once the enzyme is released at a terminator, it finds a new free factor to form a
holoenzyme to begin transcription again
-if an RNA polymerase falls off DNA prematurely, it must begin again at the
promoter
-How do the termination signals in the DNA stop the elongating
polymerase?
oTermination signals in bacterial genes consist of A-T nucleotide pairs
followed by a two0fold symmetric DNA sequence; becoming a hairpin in
RNA
The hair pin may help pull the RNA transcript from the active site
oThe DNA-RNA hybrid in the active site is held together mainly by U-A base
pairs (less stable than G-C pairs; less hydrogen bonds) is not strong enough
to hold the DNA in place, so it dissociates causing the release of polymerase
Transcription Start and Stop Signals Are Heterogeneous in Nucleotide Sequence
-Consensus nucleotide sequence derived by comparing many sequences with
the same basic function and tallying up the most common nucleotide found at each
position
oServes as an average of a larger number of individual nucleotide sequences
-Promoter strength is determined by the number of initiation events per unit time
oPromoters coding for abundant proteins are stronger than those encoding
rare proteins
-A gene typically has only one promoter, and because its nucleotide sequence is
asymmetric, the polymerase can bind in only one direction
Transcription Initiation in Eukaryotes Requires Many Proteins
-Eukaryotic nuclei have RNA polymerase I, II, and III
-RNA polymerase I transcribes 5.8 S, 18S, 28S rRNA genes
-RNA polymerase III transcribes genes encoding mainly tRNA
-RNA polymerase II transcribes most genes, including all those that encode
proteins
www.notesolution.com

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Description
Chapter 6 How Cells Read the Genome: From DNA to Protein - Proteins working closely with one another often have their genes located on different chromosomes, and adjacent genes often encode proteins that have little to do with each other - DNA uses RNA as in intermediary to direct protein synthesis o When the cell needs a particular protein, the nucleotide sequence required is first copied into RNA (transcription) and then these RNA segments are used directly as templates to direct the synthesis of the protein (translation) o DNA RNA protein From DNA to RNA Portions of DNA Sequence Are Transcribed into RNA - Step 1 is to copy a gene into an RNA nucleotide sequence (transcription) - RNA is a linear polymer like DNA made of A, U (not T), G, C nucleotides linked together by phosphodiester bonds o Differ from DNA in that the nucleotides are ribonucleotides; they contain the sugar ribose rather than deoxyribose - RNA is typically single-stranded, but may fold onto itself to have double-stranded sections Transcription Produces RNA Complementary to One Strand of DNA - Transcription begins with the opening and unwinding of a small portion of the DNA double helix - One of the two strands acts as the template for the synthesis of an RNA molecule o Nucleotide sequence of the RNA chain is determined by complementary base- pairing between incoming nucleotides and the DNA template o Incoming ribonucleotides are covalently linked to the growing RNA chain - **unlike DNA replication, RNA does not remain hydrogen-bonded to the DNA template strand o Instead, as the RNA chain forms, the DNA helix reforms - RNA molecules are considerably shorter than DNA molecules - RNA polymerases enzymes that perform transcription o They catalyze the formation of phosphodiester bonds linking nucleotides together o Unwinds DNA just ahead of the active site for polymerization o Its substrates are ATP, CTP, UTP, and GTP - Many RNA copies can be made of a single gene on a DNA strand - Differences between RNA and DNA polymerase: www.notesolution.com
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