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Summary Genetics chapters 14,15,16,17
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Chapters 14,15,16,17 genetics vu biomedical sciences
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CHAPTER 17Some genes do not encode polypeptides but are transcribed into non-coding
RNAs (ncRNAs). In most cell types, ncRNAs are more abundant than mRNAs.
In a typical human cell, only about 20% of transcription involves the production
of mRNAs, whereas 80% of it is associated with making ncRNAs (Mainly rRNAs and
tRNAs).
17.1 OVERVIEW OF NON-CODING RNAs
ncRNAs can bind to different types of molecules, which is why they can perform
so many functions.
RNA molecules can form stem-loop structures, which may bind to pockets on
the surface of proteins. The binding of ncRNAs to DNA, RNA allows ncRNAs to affect
transcription and translation.
FUNCTIONS
Scaffold: ncRNA binds a group pf proteins.
Guide: ncRNA binds to a protein and guides it to a specific site in the cell.
Alteration of Protein Function or Stability: ncRNA binds to a protein and alters that
protein's structure
the ability of the protein to act as a catalyst
the ability of the protein to bind to another molecule
the stability of the protein
Ribozyme: RNA molecules with catalytic function.
Blocker: ncRNA physically prevents or blocks a cellular process from happening.
Decoy: ncRNA recognizes another ncRNA and sequesters it.
ncRNAs are broadly categorized according to their length
• Long non-coding RNAs (lncRNAs) are longer than 200 nucleotides.
• small regulatory RNAs (also called short ncRNAs) are shorter than 200 nucleotides.
MicroRNAs fall into this category, usually 20-25 nucleotides.
The Emergence of Living Cells May Have Been Preceded by an RNA World
Researchers propose that living cells arose from more primitive structures.
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CAROLINA SANZ HERNÁNDEZ
, A protobiont is the term for a precursor to living cells which consisted of an
aggregate of molecules and macromolecules that acquired a boundary, such as a lipid
bilayer. It was able to maintain an internal chemical environment distinct from that of
its surroundings.
Characteristics that make Protobionts possible precursors of living cells:
• A boundary that separated the internal contents of the protobiont from the
external environment.
• Polymers inside the protobiont that contained information.
• Polymers inside the protobiont that had catalytic functions => simple
metabolism.
• The precursors developed the capability of self-replication.
Scientists favor the idea that RNA was the first macromolecule found in
protobionts.
RNA world refers to a period on Earth in which RNA molecules, but not DNA or
proteins, were found within protobionts.
RNA would have carried out three key functions:
1. Information storage in its nucleotide base sequence
2. Self-replication by functioning as a ribozyme and using RNA as a template to make
complementary RNA molecules
3. Catalytic activity such as the synthesis of polypeptides and other kinds of organic
molecules.
17.3 NON-CODING RNAs
ncRNAs can exert their effects on RNA molecules that are already made. ncRNAs affect
the ability of mRNAs to be translated or degraded.
Experiment 17A – mRNA silencing
Researchers had used antisense RNA to inhibit mRNA translation – complementary to
target mRNA.
• Oddly, researchers also introduced sense RNA, which also inhibited mRNA expression
• The effects of antisense RNA often persisted for a very long time (longer than
predicted based on stability of the RNA used).
Led Fire and Mello to investigate how the introduction of RNA into cells inhibits
mRNA.
THE GOAL: understand how the experimental injection of RNA was responsible for the
silencing of particular mRNAs.
Andrew Fire and Craig Mello: used the nematode worm Caenorhabditis elegans
• Easy to inject with RNA.
• The expression of many of its genes had already been established.
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CAROLINA SANZ HERNÁNDEZ
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