BIOLOGY

ORIGIN OF LIFE :PART 2

Life is a relationship among molecules and not a property of any  particular molecule(linus Pauling)

One only understands the essence of things when one knows its origin and development(Alexander oparin)

Rise of DNA 

In the protocell RNA served both as memory for coding in translation as m RNA and also took part directly in translation with help of tRNA and rRNA. Later the DNA molecule arose and transcription allowed the memory of coding proteins to reside entirely in DNA. The separation allowed independent mutation of the genetic machinery. Since there were two copies of the DNA molecule a mutation would not now disrupt the corresponding translation of protien and lead to death of the organism.

  The second advantage of DNA was that earlier during cell division the RNA could not be distributed equally to the two daughter cells. Cell division led to random distribution of genetic material between two daughter cells.Now cell division involved  mitotic separation of two DNA strands to two daughter cells allowing predictable separation of genetic material

The first Eukaryotic cell

Sometime in deep time a bacteria and archaebacteria cells fused together. The archaebacteria genes had active mechanisms for RNA cutting and ligation inherited from the RNA world. The archaebacteria genes then tried to invade the bacterial RNA and incorporate its genes there. The bacteria in an effort to prevent this synthesized a nuclear membrane which prevented ingress of archaebacteria RNA inside the membrane. RNA could now only egress through nuclear pores. This resulted in separation of transcription and translation in the composite cell. 

  Bacteria depends on gene transfer for incorporation of new genes mainly through transfer of RNA by conjugation. With the nuclear membrane this source of evolution of cell got blocked. The composite cell with nuclear membrane now utilised sexual reproduction to obtain new genes for evolution.It devised meiosis during which the DNA strands got separated into two gametes. Next two gametes from different organisms fused to form the zygote. The zygote was a new organism with new genetic material.

     Another major advancement in eukaryotic cell was ability to form diverse membranes and cytoskeleton. The loss of outer bacterial wall led to inner membrane being manipulated to form psuedopodia and to engulf another bacteria as food. These were the first animals, The engulfed food was later digested in a vesicle.Other membrane structures arose such as Golgi complex and endoplasmic reticulum for intracellular transport.

The bacterial cell can utilize energy only in its cell membrane and has limited surface area to do so. In eukaryotic cell archaebacteria exist as mitochondria inside cells and all of them have membranes which can utilize energy. A eukaryotic cell has a much greater energy reserve as a result.

An alternative view of rise of membrane synthesis capability states that the eukaryotic cell developed it de novo by mutations. Thereafter it was used for phagocytosis ,developed by ability to curve membranes,which is a new mode of obtaining nutrition and not present in bacteria .Mitochondria  in eukaryotes arose from phagocytosis of archaebacteria.

Origin of multicellular organisms

The eukaryotic cell may have not have separated after cell division in many cases and existed to live together.The entire organism could have laid on a carpet of underlying bacteria and obtained food from it. Initially the cells could have obtained food separately. Later there would have been specialization with some secreting enzymes and others distributing the nutrients. Gradual interdependence of specialized cells would give rise to the multicellular organism.

    Once a multicellular organism forms it has to undergo development from a zygote to obtain a three dimensional form. Information in DNA occurs in a linear form and this has to be converted to a three dimensional gradient of information. Secondly there has to be appropriate triggers to coordinate the modules  at certain times in development. Lastly the form has to be scale invariant, a small leaf has to be created identical in proportions to a large leaf during development.

   In multicellular organisms assymetry in cell boundaries coordinated developmental modules. A epithelial cell layer has a basal lamina which is different from the other sides.This triggers and coordinates the secretion of morphogens which will create the next module.

    Morphogens are secreted by cells in the embryo. They diffuse outward from their source and their concentration falls with distance. Cells respond and become differentiated to different concentration of morphogens,If there are multiple morphogens being secreted simultaneously a composite gradient forms across the tissue. Cells respond to a composite level now.    

   Morphogen concentration is not dependent on absolute amount of morphogen secreted by a cell. This is because of self organization by self excitement and lateral inhibition. As absolute amount of morphogen rises ,more of it is degraded near the source of production. As a result the concentration of the morphogen remains constant over the tissue in spite of wide variations in quantity. This preserves the gradient concentration irrespective of size.

Hierarchy formation 

The origin of multicellular organisms is an example of cooperation between cells. Cells will cooperate if the benefits of cooperation increase their survival. However the cells in a metazoa will get the benefit even if they do not contribute to the function of the body.Morever many cells can undergo mutation which can increase their own growth at the expense of the rest of the cells as occurs in malignancy. The metazoan s separate genes which later develop in gametes from genes in somatic cells during embryogenesis. Thus somatic mutations cannot affect future generations. Even if  lethal mutations occur in genes in gametes they do not survive till reproductive age and are eliminated .However other nonlethal genes can survive till old age and can be transmitted in future generations. The genes for atherosclerosis is an example.

Ack:life unfolding,The logic of chance,Sequence-evolution-function ,Imaginal discs