Cell Biology: DNA Lesson 1 – DNA Replication ( Inquiry into Life pg. 489-493 )
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Transcript of Cell Biology: DNA Lesson 1 – DNA Replication ( Inquiry into Life pg. 489-493 )
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
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 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 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