The coding sequence for most genes are corrupted by noncoding sequenceshence split genes
• the coding sequences are called exons for expressed sequences
• the noncoding sequences are referred to as introns for intervening sequences
• What are the implications of this?
• When transcription of a gene occurs, both introns and exons are part of the initial RNA
referred to as premRNA.
• The premRNA needs to be processed
o This is accomplished by a process known as splicing the removal of intervening
sequences and the joining of coding sequences to each other to give us the actual
mRNA that will be translated.
• Is a complex process which utilizes both protein and RNA as enzymes.
– This was the first time that researchers recognized that RNA can have enzymatic function
– RNA sometimes enzymatic reaction, pre RNA enzymatic action
– Sometimes the RNA that is spliced has itself that enzymatic activity.
• The precision of splicing is crucial because even one nucleotide error in either direction can result in a
variant coding sequence which may result in a nonfunctional protein.
• Alternative splicing of premRNA occurs in some cases, and this gives rise to a variety of proteins which
are essentially coded for by the same DNA segment. The Cell Cycle 10/16/2012
The Cell Cycle
• Describes the growth and division of cells in the embryo or in an organ or tissue that is:
• Enlarging occurs during development and growth into adulthood
Repairing occurs in the adult when certain tissues have been damaged (e.g. a scrape or cut of
• Replenishing e.g replacement of red blood cells or epithelial cells that die on a daily basis
• In the adult, most cells are in a stationary or maintenance phase
• Bacteria divide every 20 minutes
• The length of the cycle can vary, depending on the age, environmental conditions, and type of tissue.
• An overall rough estimate is about a day or 24 hours for cells that are actively in the cycle.
• The vast majority of time is spent in a growth phase known as INTERPHASE.
– Interphase is subdivided into three time periods:
• G1 a time when there is a lot of growth enlargement of the cell. (almost half of
the time 810 hours); catabolism/anabolism, enlargement of cell
• S phase the time when DNA replication takes placecell is signaled whether/
when to enter Sphase; parental strands template for new strands, about 8 hours
• Cell needs to know if mistakes can be fixed in DNA replication, process
• If mistakes are not fixable cell is likely to die
– G2 completion of certain growth aspects such as duplication of centrioles.
• The rest of the time in the cycle is MITOSIS which involves the actual division of the cell to result in two
identical cells. This process has four basic phases that can be recognized microscopically.
– Prophase The Cell Cycle 10/16/2012
• Changing of the cell morphology, nucleus disappearing nuclear envelope
disintegrating, chromatin condenses into chromosomes, centrioles duplicated,
move to opposite poles o