Lecture--12 September

 

Serial endosymbiosis not only accounts for the presence of interior organelles such as golgi, mitochondria and chloroplast, but may also account for organelles such as cilia and flagella.  Margulis believes that the flagellum may be derived anciently from some creature such as a spirochaete.

 

One of the large somewhat unanswere3d questions is the derivation of the genetic material. There are two materials associated with cellular genetic information, DNA and RNA.

 

DNA

Consists of

  1. nucleotides--adenine, thymine, guanine, cytosine, (only in messenger—uracil)
  2. sugar phosphate backbone

Takes the form of a double helix.  The DNA helix was first worked in in Wilken’s lab in London by Wilkens and Rosalind Franklin.  James Watson does not provide a very kind image of Rosalind in his book, The Double Helix.

 

RNA—typically a  non helix, but can double back on itself and do a bunch of “wild” things. There are three forms of RNA

  1. messenger—carries information from nucleus to cytoplasm        
  2. transfer—brings amino acids to the vicinity of the ribosome.
  3. ribosomal—reads the genetic code three nucleotides at a time and “bekons” the correct transfer RNA+ AA combo into the right place where a polypeptide chain can be encoded.

 

The Central Dogma of Molecular Biology is One Gene, One Enzyme. The DNA in the nucleus is translated into a single strand of mRNA, moves through nuclear pores to where the polypeptide chain is transcribed by a reading of the mRNA at the rRNA

 

Therefore, the big question again did life take place in a RNA or DNA world?  Viruses, which are non-lving entities, have either RNA or DNA cores, but required the transcription and replication process to take place in another living cell.

 

Surface area to Volume relationships

An other interesting question at this time was the question of What is the appropriate size for a cell. To answer that question, one needs to know the relationshp between volume (where metabolism takes place) and surface area (where the ingredients move either into or out of the cell).

 

Side length

Volume

Surface Area

Ratio of V:S

Equation

l x w x h

(l x w )*6

 

1

1

6

1:6

2

8

24

1:3

3

27

54

1:2

 

It appears that the typical cell is about 1 mm in diameter. One should realize that cells are not just bags of stuff that is homogenously mixed. Materials are compartmentalized by the endoplasmic reticulum

 

How are new cells produced

 

Cellular replication includes both mitosis + cytokinesis. This process procduces cells that have the same genetic material as their parents, in equal amount.

 

Cellular production for reproduction includes meiosis + cytokinesis. This process reduces the genetic material by 1/2, so that when fertilization takes place the full complement of genetic material is restored.

 

In protists, the process of cell division is normally called binary fission

 

Next question.  If there is a functional cell size of 1 mm diameter, how can an organism increase its size without increasing the cell size. The answer to that question is multicellularity, where cells remain small, but the cell mass grows in size. Multicellularity

allows for a division of labor