Warm-up- 5 minutes
Quietly and on your own, answer the
following questions in your notes for
today. Be prepared to answer.
1. In what part of the cell cycle does
DNA replication occur?
2. Define chromosome.
3. Define nucleotide.
Homework & reminders
1. Chi-square lab write-up due
TOMORROW in the beginning of
class.
1. ALL late write ups = automatic 50% and additional 10%
points for each day that it’s late. No excuses will be
accepted.
2. Chapter 16 quiz on Wednesday.
Start studying TODAY.
Today’s Agenda
1. Warm up
2. Chi-square lab overview. Due tomorrow.
Worksheet is bonus.
3. Review midterm exams.
4. Chapter 16 lecture.
5. Start DNA replication diagram.
Chapter 16: The Molecular
Basis of Inheritance
Chapter 16 vocabulary list
List of need to know words are provided on the class
wiki page. This is for you to work on for yourself.
Building a Structural
Model of DNA
• After most biologists became convinced that
DNA was the genetic material of life, the next
challenge was to determine its structure.
• Rosalind Franklin produced a picture of the
DNA molecule by using a technique called Xray crystallography Franklin produced a
picture of the DNA molecule using this
technique
LE 16-6
Rosalind Franklin
Franklin’s X-ray diffraction
photograph of DNA
• Based on the images, two other scientists
named Watson and Crick were able to
determine that DNA molecules took a double
helix shape.
LE 16-7
5 end
Hydrogen bond
3 end
1 nm
3.4 nm
3 end
0.34 nm
Key features of DNA structure
5 end
Partial chemical structure
Space-filling model
Sugar–phosphate
backbone
Nitrogenous
bases
5 end
Thymine (T)
Adenine (A)
Cytosine (C)
Phosphate
Sugar (deoxyribose)
3 end
DNA nucleotide
Guanine (G)
• Watson and Crick built models of a double
helix to match to the X-rays and chemistry of
DNA
o The side strands, or “backbones” of the DNA
molecule are made of a sugar (deoxyribose)
paired with a phosphate.
o The deoxyribose backbones are joined together by
a series of molecules called nitrogenous bases.
Nitrogenous Bases
• There are two types of nitrogenous bases:
o Purines
• Much wider
• Include adenine and guanine
o Pyramidines
• Much narrower
• Include cytosine and thymine
LE 16-UN298
How do the four bases
combine to form DNA?
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
matches data from X-rays
• Watson and Crick
reasoned that the
pairing was more
specific –
o Adenine paired only
with Thymine
o Guanine paired only
with Cytosine
Base Pairing to a Template
Strand
• DNA is a double-helix molecule made of two
intertwining strands.
• The two strands of DNA are complementary,
meaning each has a set of bases that
corresponds with the other.
• In DNA replication, the molecule is be
separated into its two strands.
o Two new strands can be made from these
templates, duplicating the molecule.
LE 16-9_1
The parent molecule has
two complementary
strands of DNA. Each base
is paired by hydrogen
bonding with its specific
partner, A with T and G with
C.
LE 16-9_2
The parent molecule has
two complementary
strands of DNA. Each base
is paired by hydrogen
bonding with its specific
partner, A with T and G with
C.
The first step in replication
is separation of the two
DNA strands.
LE 16-9_3
The parent molecule has
two complementary
strands of DNA. Each base
is paired by hydrogen
bonding with its specific
partner, A with T and G
with C.
The first step in replication
is separation of the two
DNA strands.
Each parental strand now
serves as a template that
determines the order of
nucleotides along a new,
complementary strand.
LE 16-9_4
The parent molecule has
two complementary
strands of DNA. Each base
is paired by hydrogen
bonding with its specific
partner, A with T and G
with C.
The first step in replication
is separation of the two
DNA strands.
Each parental strand now
serves as a template that
determines the order of
nucleotides along a new,
complementary strand.
The nucleotides are
connected to form the
sugar-phosphate backbones of the new strands.
Each “daughter” DNA
molecule consists of one
parental strand and one
new strand.
Origins of Replication
• Replication begins at special sites called
origins of replication.
o The two DNA strands are separated, opening up a
replication “bubble”
o Each chromosome may have hundreds or even
thousands of origins of replication
o Replication proceeds in both directions from each
origin, until the entire molecule is copied
LE 16-12
Parental (template) strand
Origin of replication
Bubble
Daughter (new) strand
0.25 µm
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at may sites
along the giant DNA molecule of each chromosome.
In this micrograph, three replication
bubbles are visible along the DNA
of a cultured Chinese hamster cell
(TEM).
Elongating the DNA Strand
• Enzymes called DNA polymerases catalyze the
elongation of new DNA.
• The rate of elongation is about 500 nucleotides per
second in bacteria and 50 per second in human cells.
Proofreading and
Repairing DNA
• DNA polymerases also proofread newly made
DNA, replacing any incorrect nucleotides.
• Two types of repair:
o In mismatch repair, the enzymes replace incorrect
bases with the correct ones.
o In nucleotide excision repair, enzymes cut out and
replace entire stretches of DNA that are damaged.
Replicating the Ends of
DNA Molecules
• DNA polymerase has one significant limitation.
• The enzyme has no way to complete one of
the ends.
o Every time the DNA is copied, it becomes a little
shorter.
• Cells will divide countless times over the
lifespan of an organism. How can DNA be
protected, given this limitation?
• Eukaryotic chromosomal DNA molecules
have at their ends repeating nucleotide
sequences called telomeres.
o Telomeres are DNA, but do not actually encode
for any traits.
o Telomeres do not prevent the shortening of DNA
molecules, but they postpone it.
• Eventually, the telomeres are worn
down and essential genes begin
to be lost from the chromosomes.
o This is one of the hypothesized causes
of aging.
• An enzyme called telomerase
catalyzes the lengthening of
telomeres in stem cells.
o This enzyme cannot be produced
indefinitely due to an increasing risk of
the cell growing uncontrollably
(cancer)
Chromosome and
chromatin
Activity 1: DNA
replication
1. Draw figure 16.13 on page 314 and include the
four main blue labels showed in the diagram, the
RNA primer and label the 3’ and 5’ ends.
2. Draw figure 16.15 and include the 2 main blue
labels. Also label the origin of replication, RNA
primer, sliding clamp, DNA pol III, parental DNA,
and the 3’ and 5’ ends.
3. Draw figure 16.16 and label the steps of
synthesizing the lagging strand.
4. Summarize DNA replication starting from unwinding
of the DNA to the end of synthesizing the lagging
strand. Include the important proteins and
enzymes. Include the appropriate 3’ and 5’ labels.
Activity 2: DNA &
Chromosomes
1. Draw one nucleotide using the figure 16.5 on page 308 for
reference. In your diagram, include the labels: nucleotide,
phosphate, deoxyribose, and nitrogenous base
2. Draw figure 16.7 A on page 309. Include the labels: double
helix, nitrogenous bases, adenine, guanine, thymine, and
cytosine.
3. Diagram chromatin packing in shown in figure 16.22 on page
321. Include in your diagram the labels: DNA, double helix,
histones, chromatin, and chromosome
4. When you are done drawing, you will write a comprehensive
story about the molecular unit of inheritance by including all
of the words above.