Living Textbook MC610

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Introduction to Molecular Biology

Historical Perspective:

For a long time people wondered how races and species stay intact. For example, bacterial cells in fossils have the same basic enzymes and proteins as those present today. Moreover the basic structure of a bacterial cell is very similar to a mammalian cell.

Two scientists really set the tone for our understanding of these questions:

Mendel , who was working on peas, and stumbled upon an observation that if you cross wrinkled peas together you get wrinkled peas, while if you crossed wrinkled and round peas you get a mixture, but if you cross only round peas together, you sometimes get wrinkled peas:

    W + W = W

    W + R = W + R

    R + R = R + W

    He proposed that characteristics pass from one generation to another via "inheritance units", and that some are dominant while others are recessive. Johansen in early twenties century gave these units their modern name: Genes.

Darwin, who at about the same time and unaware of the observations of Mendel, was studying evolution and proposed that all living beings come from a common ancestor through "inheritance units".

So what are those inheritance units or genes?

The first experiment to answer this question was simple: Sacrifice a chicken, homogenize, what do you get? Cold chicken soup. The cells in this soup were viable, i.e. used oxygen.

Now boil your homogenate and what do you get: hot chicken soup, but also now the cells are non-viable, do not use oxygen.

So what are the inheritance units responsible for life: Proteins.

In 1889, Kossel identified nucleic acids and then others followed and identified DNA and RNA.

The general consensus among scientists was that proteins were the inheritance units still and these newly discovered nucleic acids have another yet to identify functions.

Why? Units, nucleic acid you have four bases. Proteins, you have twenty amino acids. A much higher combination and thus account for the variations in characteristics.

Griffith then performed a simple experiment. He found two types of the pneumococcus sp., one with a capsule (Called it S for smooth) and another without (R for rough). The S-form is virulent while the R is not.

He took the S-form, added it to the R-form...Virulent

He took the S-form, boiled it added it to R-form...Non-virulent.

He took the S-form, heated it to 50°C and added to R-form...Virulent

So the inheritance units cannot be proteins, but rather nucleic acids.

Oswald Avery isolated DNA and were then able to positively identify it as the inheritance units.

Hershey also confirmed these observations by using a bacteriophage that is labeled with 32P for nucleic acids, and 35S for proteins, and found that only 32P ended up in the bacterial cells.

Chargaff found out that in any nucleic acid A = T and G = C.

Rosalind Franklin did an X-ray to show the double helix of DNA.

In 1953, Watson and Crick took all this body of work and published one of the most famous papers in history that detailed the structure and properties of DNA, which turned out to be very accurate.

Important Notes:

Bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T) and Uracil (U).

Nucleoside is a base plus a sugar (ribose in RNA, and deoxyribose in DNA).

Nucleotide is a nucleoside plus a phosphate molecule.

DNA is arranged as a phosphoribose backbone (primary structure), then two complementary strands come together, joined through base pairing (which are really hydrogen bonds) and hydrophobic interactions between the bases, to form the secondary structure, the double helix. Then the helices are packed into a tertiary structure where the DNA is still accessible.

Each cell contains approximately 2 meters of DNA. With about 1014 cells in our bodies, that gives us 2 x 1011 km. The distance from sun to earth is 1.5 x 108 km.

Tertiary structure involves packing DNA on proteins known as histones. In addition, negative supercoiling of the double stranded DNA allowing for packing while each strand is still accessible for vital processes. This step is carried out by a group of enzymes known as DNA Topoisomerases (or Gyrases). These enzymes form a nick in the DNA, allow negative supercoiling and then reseal the nicks.

The DNA is an important document that contains all the traits of life. It is well protected inside the nucleus, and can never leave there. But all process take place outside the nucleus, so how does the body conduct this?

DNA can be considered as a recipe book that is placed in a safe. mRNA is the cook who is allowed to come and copy one specific recipe (transcription), take it out to the kitchen (ribosomes) and uses the ingredients (tRNA and proteins)to cook (translation) the meal (enzyme or protein). If we need to open another restaurant (cell), the recipe book (DNA) is first copied with utmost accuracy (replication), with a system of proofreading and recognition of mistakes that are either corrected or the whole book is destroyed.