NUCLEIC ACIDS, continued...

DNA is DOUBLE STRANDED. A twisted Double Helix.
  The opposing strands in DNA are ANTI-PARALELL.
    What does that mean? Why is each strand considered to be POLAR in nature?
       That is, each strand has inherent polarity. Why?
What is meant by opposing bases in DNA being COMPLIMENTARY?
Why are hydrogen bonds important in the double helix?



  RNA is mostly single stranded.
  U substitutes for T in RNA.

ALMOST universally, DNA carries the master set of genetic instructions in organisms.

    E.g., in all eukaryotes AND in mitochondria and chloroplasts (remember, they are semi-autonomous and have their own DNA).

RNA acts as an intermediate in the synthesis of proteins encoded by DNA.
 In some cases, RNA acts as master instructions.
     E.g.  HIV, influenza viruses

 
CELL DIVISION

A key event in the life of any organism.
 Cells grow by absorbing substances from the environment, making  polymers, organelles etc., increasing
volume (in plants, uptake of salts and water in vacuole).
 Cells enlarge until surface/volume ratio becomes too small, thereby limiting exchange of nutrients.

So, cell divides.
 Division usually necessitates both division of the nucleus and division of the cytoplasm.
  Called KARYOKINESIS and CYTOKINESIS.
    Mitosis is a term generally used to describe all of cell division, but
        it specifically is equivalent to just karyokinesis

Another way to look at cell division:
 In multicellular terrestrial plants, growth necessitates increase in cell number
  in order to provide the raw material for growth,  i.e. cell expansion or increase in volume.

Cells undergo a fairly regular cycle of growth and division.
 Called THE CELL CYCLE.   (Fig. 3.16)

In a very broad sense, the cell cycle consists of two parts:
 Interphase and division.
  During interphase, cells metabolize and grow.
   Interphase lasts 12-30 hours in plants, on average.
   Division lasts around 2 hours.

Interphase and division are subdivided into distinct subphases.

Interphase divided into G1, S, G2.
   (G means ‘gap’)
 G1 --- cell metabolizes, makes polymers and organelles, and grows.
   S --- nuclear DNA REPLICATES. Master set of instructions duplicates. Why?
          We’ll see how DNA replicates later.
 G2 --- having completed DNA replication, cell prepares to divide by making more molecules etc.

 M --- division
 

The cell cycle is very tightly regulated, by MANY PROTEINS AND GENES.  

CHECKPOINTS are places in the cycle in which regulatory proteins check whether the cell cycle has progressed with
        fidelity.
  E.g., ‘Start’, at the end of G1, controls entry into S.
   Also a checkpoint at entry into M. Cell senses if DNA replication is complete.
            Defects in these checkpoints can be disastrous.   Why?

There are common features to regulation of cell cycle in all eukaryotes.
    Why?


Genuine logistical problems in dividing a cell. Why?

 Average human cell is around 30 um in diameter, with a 10 um nucleus.
 Within the nucleus is 10 FEET of DNA, consisting of  46 separate, very thin threads.
 When duplicated, about 20 feet of DNA!

If the cell was 1 foot in diameter, the DNA would be 20 miles long!!!

So, we have a DNA packaging and sorting problem.
   The solution?
 DNA associated with histones as chromatin --   chromosomes can coil up and effectively shorten.
     I.e., DNA divided into units, or chromosomes -- more managable.