BIO130H1 Lecture Notes - Lecture 7: Transcription (Genetics), Deoxyribose, Ribose

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Published on 2 Mar 2011
School
UTSG
Department
Biology
Course
BIO130H1
Professor
Lecture 7 Notes
Chapter 6: How Cells read the Genome: From DNA to Protein
When the cell needs a particular protein, the nucleotide sequence of the appropriate portion of the
DNA is first copied into RNA transcription.
It is these RNA copies of segments of the DNA that are used directly as templates to direct the
synthesis of the protein translation
The flow of genetic information in cells is therefore from DNA to RNA to protein. All cells express
their genetic information in this way—a fundamental principle called the
central dogma of molecular biology.
Portions of DNA Sequence Are Transcribed into RNA
Like DNA, RNA is a linear polymer made of four different types
of nucleotide subunits linked together by phosphodiester bonds. It
differs from DNA chemically in two respects:
1.The nucleotides in RNA are ribonucleotides—that is, they contain the sugar ribose rather than
deoxyribose
2.Although, like DNA, RNA contains the bases adenine (A), guanine (G), and
cytosine (C), it contains the base uracil (U) instead of the thymine (T) in
DNA. Since U, like T, can base-pair by h- bonding with A
3.DNA is a ds-helical molecule, whereas RNA is ss (but can fold into a ds
shape)
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 to
expose the bases on each DNA strand, where one strand behaves as a template
The nucleotide sequence of the RNA chain is determined by the complementary base-pairing
between incoming nucleotides and the DNA template.
When a good match is made, the incoming ribonucleotide is covalently linked to the growing RNA
chain in an enzymatically catalyzed reaction. The RNA chain produced by transcriptionthe
transcriptis therefore elongated one nucleotide at a time
oIts nucleotide sequence is exactly complementary to the strand of DNA used as the template
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Transcription differs from DNA replication in several crucial ways
1.Unlike a newly formed DNA strand, the RNA strand does not remain hydrogen- bonded to the
DNA template strand. Instead, the RNA chain is displaced and the DNA helix re-forms. Thus,
the RNA molecules produced by transcription are released from the DNA template as single
strands.
2. In addition, because they are copied from only a limited region of the DNA, RNA molecules are
much shorter than DNA molecules.
The enzymes that perform transcription are called RNA polymerases-- it catalyzes the formation
of the phosphodiester bonds that link the nucleotide together to form a nuclear chain.
oIt moves stepwise along the DNA, unwinding the helix to expose a new region of the template
strand for complementary base-pairing.
oIn this way, the growing RNA chain is extended by one nucleotide at a time in the 5-to-3
direction.
o The substrates are nucleoside triphosphates (ATP, CTP, UTP, and GTP; the hydrolysis of high-
energy bonds provides the energy needed to drive the reaction forward
oThe immediate release of the RNA transcript allows for many copies to be madeesp. with a
speed of 20 nucleotides/sec, thousands of copies can
be made
DNA polymerase vs. RNA polymerase
1.RNA polymerase catalyzes the linkage of ribonucleotides, not
deoxyribonucleotides
2.RNA polymerases can start an RNA chain without a
primer as transcription isnt as accurate as DNA
replication
3.Unlike DNA, RNA does not permanently store genetic information in cells. RNA polymerases
make about one mistake for every 104 nucleotides copied into RNA
4.X-ray crystallographic studies of both types of enzymes reveal that, other than containing a
critical Mg2+ ion at the catalytic site, they are virtually unrelated to each other
Template-dependent nucleotide polymerizing enzymes seem to have arisen independently
twice during the early evolution of cells. One lineage led to the modern DNA polymerases,
and the other led to the modern cellular RNA polymerases
Yet, it still has a modest proofreading system:
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Document Summary

Chapter 6: how cells read the genome: from dna to protein: when the cell needs a particular protein, the nucleotide sequence of the appropriate portion of the. All cells express their genetic information in this way a fundamental principle called the central dogma of molecular biology. Portions of dna sequence are t ranscribed into rna: like dna, rna is a linear polymer made of four different types of nucleotide subunits linked together by phosphodiester bonds. Since u, like t, can base-pair by h- bonding with a: dna is a ds-helical molecule, whereas rna is ss (but can fold into a ds shape) Instead, the rna chain is displaced and the dna helix re-forms.  template-dependent nucleotide polymerizing enzymes seem to have arisen independently twice during the early evolution of cells. In eukaryotes, a transcription unit typically carries the information of just one gene. Dna template as a guide: a detachable subunit called sigma (