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.