Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Excerpt from “Biology: Boundless Biology Open Textbook”, CC BY SA 3.0

https://www.boundless.com/biology/dna/
Introduction to DNA

DNA is the molecule responsible for inheritance.

During the first half of the 20th century, genes were known to exist on chromosomes. However, chromosomes are composed of both protein and DNA. Scientists did not know which of these was responsible for inheritance. Through a series of experiments between 1928 and 1952, scientists provided evidence that DNA was the molecule responsible for inheritance. The details of these experiments are discussed in this chapter.

James D. Watson and Francis Crick determined the structure of DNA in 1953 by using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA had a helical structure (i.e., shaped like a corkscrew in the above figure). Their double-helix model had two strands of DNA with the nucleotides pointing inward. Each strand matched a complementary nucleotide on the other strand to form what looked like rungs on a twisted ladder. This structure showed that genetic information exists in a sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for duplication. If the strands are separated, new partner strands can be reconstructed for each, based on the sequence of the old strand.

We now know much more about genes. They are the discrete units of heredity contained within the DNA double helices of chromosomes. A gene is a specific stretch of DNA and RNA that codes for a type of protein, or for an RNA chain, that has a function in the organism. Living things depend on genes because they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring. All organisms have many genes corresponding to various different biological traits. Some of these traits are immediately visible, such as eye color or number of limbs. However, some of these traits are not immediately apparent, such as blood type, increased risk for specific diseases, or the thousands of basic biochemical processes that comprise life.

In the time since Watson and Crick first published the DNA structure, we have learned that DNA replication, or the copying of DNA, is a biological process that occurs in all living organisms and is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule. Each strand of the original double-stranded DNA molecule serves as a template for the production of the complementary strand. Cellular proofreading and error correcting mechanisms ensure a near perfect result for DNA replication.