DNA: STRUCTURE AND REPLICATION

DNA: The Code of Life  DNA is the molecule that contains all of the hereditary material for an organism  It is found ONLY in the nucleus of eukaryotic cells  DNA is also a nucleic acid, which means it is composed of thousands of subunits called nucleotides  Each nucleotide in DNA consists of  A phosphate group  Deoxyribose (5-carbon sugar)  A nitrogenous base

It’s a Double Helix!  In 1953, Dr. James Watson and Dr. Francis Crick developed a model of the DNA molecule that showed DNA is a “double helix,” or a twisted ladder  Discovery of the Double Helix Discovery of the Double Helix

Composition of the Ladder  The sides of the “ladder” are formed by a backbone of deoxyribose and phosphate molecules  The “rungs” consist of 2 nitrogenous bases joined together in the middle by hydrogen bonds Phosphate Sugar

Meet the Nitrogenous Bases!  The nitrogenous bases that make up the “rungs” of the DNA ladder are:  Adenine (A)  Thymine (T)  Cytosine (C)  Guanine (G)  These bases are the “letters” that spell out the genetic code

Base Pairing  Two bases (a pair) make up each “rung” of the DNA ladder  In base pairing:  Adenine always pairs with Thymine (A-T)  Cytosine always pairs with Guanine (C-G)

The Formation of DNA  To make DNA, nucleotides are joined together in long, continuous strands of nucleotide building blocks  Two DNA strands bind together to form the “double” helix  The binding of two DNA strands to each other requires that the bases pair in a very controlled and restricted manner

Complimentary Strands!  An A on one strand means that the opposite strand must contain a T at the same position  A G on one strand means that the opposite strand has a C at the same position  In this way, the strands are said to be complimentary!

Determine the Complimentary Strand!  The following sequence of bases is found on one strand of DNA: ATGCCAATGGGATC  What is the sequence of bases on the complementary strand? ATGCCAATGGGATC TACGGTTACCCTAG

Practice Problem  A strand of DNA has 100 bases. Thirty-five of these bases are adenine. How many bases are cytosine? Adenine and Thymine are complimentary, so if 35 adenines are present, 35 thymines must also be present = 70 bases 100 – 70 = 30 bases leftover to represent cytosine and guanine Cytosine and guanine are complimentary, so equal numbers of both bases should be present. 30/2 = 15 Cytosine Bases

DNA Replication  We have learned that during mitosis, chromosomes duplicate. This means that DNA must be able to make a copy of itself!  To replicate the genetic material, the two strands of DNA are separated one base at a time and a new strand of DNA is made using the old strand

DNA Replication  DNA literally unwinds and “unzips” along the weak hydrogen bonds that hold the base pairs together  Once the DNA “unzips,” free nucleotides floating in the cytoplasm pair with the “free” bases of both DNA strands  Because the nitrogenous bases can only pair in one way (A  T and C  G), two new double strands of DNA are formed  These two new strands are identical to each other and the original DNA molecule Replication AnimationSecond Animation