Cell Biology: DNA Lesson 1 – DNA Replication ( Inquiry into Life pg )

Today’s Objectives  Describe DNA Replication, including:  Describe the three steps in the semi-conservative replication of DNA:  “unzipping” (DNA helicase)  Complementary base pairing (DNA polymerase)  Joining of adjacent nucleotides (DNA polymerase)  Describe the purpose of DNA replication  Identify the site of DNA replication within the cell

Background information  It is important to recall the information we learned about DNA from the last 2 units:  DNA stands for deoxyribonucleic acid  DNA is the molecule that makes up chromosomes and serves as hereditary information

Nucleic Acids  There are two types of nucleic acids, DNA and RNA  Both DNA and RNA are polymers of nucleotides (chains of joined nucleotides)  They form genetic material and are involved in the functioning of chromosomes and protein synthesis  DNA Structure:  The shape of DNA is a double helix made up of repeating nucleotide units

Nucleotide Structure  Nucleotides are composed of 3 parts:  1) phosphate molecule  2) 5 carbon sugar  Deoxyribose in DNA  Ribose in RNA  3) Nitrogen containing base

Nitrogen containing bases  Two types:  Purines – adenine (A) and guanine (G)  Pyrmidines – thymine (T), uracil (U) and cytosine (C)  DNA has A,G,T,C, RNA has A,G,U,C  Purines are larger than pyrimidines and have a double ring structure

Nitrogen containing bases  Pyrimidines are smaller than purines and have a single ring structure

When the bases bond together to form the “rungs of the DNA ladder” they do so in a set pattern. The alternating sugar and phosphates make up the rails (backbone). The bases make up the rungs.

Complementary Base Pairing  Adenine always bonds to thymine  Two hydrogen bonds  Guanine always bonds to cytosine  Three hydrogen bonds  This bonding of bases is called complementary base pairing  They cannot bond any other way because 2 purines would overlap and 2 pyrimidines would be too short to form the rungs of the ladder

The double strand is held in place by hydrogen bonds between the bases. It is the number and order as well as the type of the bases that determine what kind of organism will develop.

Order of Bases  Example:  ATCCGATT means something entirely different than ACCGTTAT, just as the words hate and heat mean different things even though they contain the same letters  As a DNA strand lengthens, it twists into a double spiral called a double helix

Candy DNA Model  Next class you will build a DNA molecule using CANDY! (you can eat it after you are finished)  DNA model… DNA model…  …. ….

Functions of DNA  1) Replicates (duplicates) itself so each new cell has a complete, identical copy  2) Controls the activities of a cell by determining which proteins are produced  The combination of proteins determines the characteristics of each living organism  3) Undergoes occasional mutations (mistakes in replication) which accounts for the variety of living things on earth

DNA Replication  Cool website demonstrating the steps in DNA replication 

 Steps in DNA Replication 1. The DNA molecule becomes untwisted by the enzyme DNA helicase breaking the H-bonds. The two strands that make up DNA become unzipped. (ie. The weak hydrogen bonds between the paired bases are broken by the DNA helicase) Each side acts as a template.

2. New complementary nucleotides, always present in the nucleus, move into place and pair with complementary bases on the exposed strands. T joins to A C joins to G

3. The adjacent nucleotides, through their sugar-phosphate components become joined together along the newly forming chain. The enzyme DNA polymerase helps this. (Adds nucleotides to template).

4. When the process is finished, 2 complete DNA molecules are present, identical to each other and to the original molecule.

5. Both DNA will now wind back up into their helical shape.  DNA replication is called semi-conservative because each new double helix is composed of an old (parental) strand and a new (daughter) strand.  Each strand conserves a length of the old strand  Enzymes assist the unwinding process, join together the nucleotides, and assist the rewinding process and many others.  When errors are made in replication. A mutation can arise.

Meet the enzymes involved in DNA replication  DNA presentation DNA presentation