Cell Biology:DNA

Lesson 1 – DNA Replication (Inquiry into Life pg. 489-493)

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… ….

Functions of DNA 1) Replicates (duplicates) itself so each new

cell has a complete, identical copy 2) Controls the activities of a cell by

producing proteins 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 http://

www.pbs.org/wgbh/aso/tryit/dna/shockwave.html

Steps in DNA Replication

1.     The DNA molecule becomes untwisted by the enzyme DNA helicase breaking the H-bonds. The 2 strands that make up DNA become unzipped. Each side acts as a template. (ie. The weak hydrogen bonds between the paired bases are broken by an enzyme)

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