Biology 120 Notes (Part 21) Continuing the Discussion on Mutations and Starting Transcription, RNA Processing, and Translation

More About Mutations

• Mutations can have different impacts on fitness.

  1. ==Beneficial mutations== increase the fitness (ability to survive and reproduce) of an organism.
  2. ==Neutral mutations== do not affect an organism’s fitness.
  3. ==Deleterious fitness== decreases the fitness of an organism.

• Most point mutations are neutral or deleterious.

• Mutations can be spontaneous or induced.

• ==Induced mutations== are caused by mutagens (environmental factors that alter base composition of DNA.

• ==Cancer-causing mutagens== are called carcinogens.

• Examples of carcinogens include tobacco smoke, ethylene dibromide, and some pesticides.

Transcription

• ==Transcription== is the first step of protein synthesis and occurs in the nucleus.
• DNA is the template and is catalyzed by the enzyme polymerase. (The link for how DNA is catalyzed by the enzyme polymerase can be located here. https://www.britannica.com/science/DNA-polymerase
• ==Transcription occurs in three steps which are: initiation, elongation, and termination.==
• ==Initiation== occurs when RNA polymerase binds to a promoter.
• A ==promotor== is a region of DNA that defines the start of transcription, the direction of transcription, and which strand of DNA will be transcribed.
• In ==elongation==, RNA polymerase moves down the DNA template strand in the 5’ to 3’ direction until it reaches a stop sequence.
• In ==termination==, the stop sequence causes the RNA polymerase to release the newly formed RNA strand.
• A newly created RNA strand is called a ==transcript==.

RNA Processing

• The newly formed RNA transcript must undergo processing before it can leave the nucleus.
• Before processing, the strand is referred to as a pre-mRNA.
• A ==pre-mRNA== contains exons and introns.
• Modifications need to be made to pre-mRNA before it leaves the nucleus.
• ==These modifications include a modified guanine nucleotide called a cap being placed at the 5’ end, a chain of 150-200 adenine nucleotides being added to the 3’ end (this is called a poly-A tail), and introns must be excised by complexes called spliceosomes.==

Translation

• ==Translation== takes place in the cytoplasm.
• The sequence of codons is read by a ribosome and is translated into a polypeptide.
• ==Translation involves all three types of RNA (mRNA, tRNA, and rRNA).==

Transfer RNA

• tRNA molecules transfer amino acids to the ribosomes.
• There is at least one tRNA molecule for each kind of amino acid.
• A tRNA with an attached is called a charged tRNA or aminoacyl tRNA.
• An amino acid is bound to the 3’ end of the molecule.
• The opposite end of the molecule contains an anticodon.
• In each cell, a large pool of charged tRNA’s exists in the cytoplasm.

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