Molecular Genetics DNA and Evolution

DNA controls the life of the cell by controlling the manufacture of proteins and is the material of the genes that are integral pans of the chromosomes. These proteins are used either to make other molecules or to form part of the structural network of the cell. DNA even makes the proteins that control the functions of DNA itself as well as controlling heredity. Watson and Crick are credited with first expressing the structure of DNA. You will find test questions on both the structure and function of DNA 215 well as RNA and the molecules involved in protein synthesis. DNA is now known to be a double helix in its non-replicating state.

The code for life resides in the sequence of nucleotides along the length of the molecule. The foundation of evolution is change that comes about through the interaction of an organism’s unique cellular DNA with the organism’s environment. For this reason, we include material on evolution coupled with the material on DNA. Since the function of DNA is to pass on traits from generation to generation. and we have explored heredity, we now explore the details of how this information is passed on. As we have stated before, the student should be familiar with the material outlined in this book and related material as well. We begin with the structure of DNA.

DNA

DNA is a biochemical molecule responsible for passing on the traits of an organism, which include controlling the biochemical nature and function of that organism. DNA is a double helix molecule that achieves a great deal of compacting in the nucleus by coiling in on itself many times. DNA makes RNA, which carries the code to the cytoplasm. There, ribosomes are utilized in making polypeptide chains that are proteins. Both the lack of an oxygen on the ribose sugar-thus the name deoxyn’bose nucleic acid-and the presence of the base thymine serve to prevent DNA from leaving the nucleus.

Nucleotides

The building blocks of DNA, nucleotides are used to make up the repeating units in the strands of DNA that represent the genetic code. A nucleotide consists of a sugar. a phosphate. and a nitrogen base. Since there are only four bases. researchers postulated that the enormous amount of genetic variation on the planet had to be in the sequence of the nucleotides within the DNA molecule. This sequence then controls the synthesis of precise proteins in the sequence of amino acids. On one end of the DNA molecule, the S-carbon sugar has a phosphate attached and is known as the 58 end. On the other end of the sugar is an OH that is identified as the 38 end of the DNA molecule.

Bases

Four nitrogenous bases make up the bonding sites along the center of the DNA molecule and are bonded to a carbon on the sugar molecule. There are two purines and two pyrimidines. The purine names end in -nine.

A. Adenine A purine, double-ring base with two bonding sites

B. Thymine A pyrimidine. single-ring base with two bonding sites

C. Cytosine A pyrimidine, single-ring base with three bonding sites


Histories Histories are proteins that help protect the DNA molecule. The DNA molecule is surrounded by eight or nine histones to help it form a protective DNA-histone complex in a tight space in the nucleus.

Base-Pairing

Base-pairing is the pairing of complimentary bases along the DNA strand. The sum of the bonds and the coiling of DNA makes the molecule securely attached along its entire length. Analysis of the assays of the DNA from a variety of organisms caused Erwin Chargaff to note that the percentage of adenine was almost identical to the percentage of thymine in the DNA of a cell. A similar relationship was discovered for cytosine and guanine. This was later referred to as Chargaff’s rule and led Watson and Crick to the ultimate conclusion that the molecule was directed inward-Abe result of which was their proposal that DNA was a double helix. Linus Pauling, who did so much work with proteins, wrestled with the molecule being directed outward; many others did as well, based on the strength of Dr. Pauling’s reputation.

Adenine-Thymine This purine-pyrimidine bonding is the result of each base having two hydrogen bonding sites. Adenine and thymine can bond only with each other in DNA.

Cytosine-Guanine This purine-pyrimidine bonding in the DNA molecule is the result of these bases having three sites for hydrogen bonds. Cytosine and guanine can only bond with each other.

H1 bonds The bonds between the adenine-thymine and cytosine-guanine classes of chemicals that form the base sequences in DN A, in addition to other places where we find hydrogen bonds.


Complementary Strands

Sour adenine at“ an bonds with thymine and guanine always bonds with cytosine. if one strand at the DNA molecule is known. then the other. complimentary strand can be known. This means that if )ou have one half of the molecule. you can construct the other hall, which is exactly what DNA does during replication. The main strand is used as a template to produce its complement. The nature of the bonds along the strand make the adenines on one strand line up in the opposite direction ofthe adenines on the other strand. As a result, the molecule is said to be anti-parallel.

5' end

”Hanging out“ on one end of the base pairs is the phosphate group, which is the end that starts the ”reading" of the molecule when it is being replicated. The direction of replication along the master strand then is from 58 end to 38 end.

3’ end

Opposite the base on the master strand with its 58 end is the complimentary strand with its 38 end oriented outward at the “beginning” of this strand, in an anti-parallel way.

Replication

A combination of the words “reproduce” and “duplicate,” replication refers to the act of DNA making a copy of itself. This precedes mitosis or meiosis. Mitigated by enzymes, it proceeds as two concurrent events. one from the 58 end of what is called the leading strand and the other from the 38 end of the lagging strand. The result of replication is said to be semi-conservative, since we end up with half the original DNA in each of the resulting new strands.

DNA helicase

DNA helicase is an enzyme that begins the unraveling of the DNA molecule at the sites of the hydrogen bonds.

DNA polymerase

This enzyme arranges the new nucleotides next to their complimentary base to make the new strand of DNA. As the name suggests. it makes a polymer out of individual nucleotides.

Leading strand

A leading strand is a strand of DNA that starts at the 58 end and is made continuously. It is not ‘ named for the fact that it starts first, but for the fact that, since it is made continuously, the construction of it proceeds faster.

Lagging strand

A strand of DNA that starts at the 38 end. lts production proceeds slower than the leading strand because it is made in pieces that are then bonded to the template to which it will be Complimentary. These pieces, known as Okasaki fragments, are bonded into place by DNA ligases.

RNA-RNA

RNA-RNA is a 5-carbon sugar that possesses an extra oxygen atom [compared with DNA] and replaces thymine with the base uracil, thus enabling it to pass through the nuclear envelope and lake the code of DNA to the cytoplasm. Three types of RNA are made in the nucleus and reunite in the cytoplasm in the process known as protein synthesis.






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