Chapter 11 Gene Action
DNA Structure and Replication
Why is DNA important to your life?
• DNA contains information that is critical to the
structure and function of your body’s cells
– New cells that are made in your body must contain a full
set of DNA.
– The DNA must be transferred accurately.
– The instructions encoded in DNA play a major role in
determining how your body operates.
– DNA instructions preserve many of the characteristics of a
species.
– A child’s life depends on the accurate transmission of
genetic information form his or her parents.
– DNA is the molecular basis of reproduction (of cells and
whole organisms).
Replication
• DNA is required for the building, maintenance, and regulation of all
living organisms’ cells.
• DNA molecules are packaged tightly in chromosomes. (The DNA is
wrapped around proteins to help it pack together tightly) Figure
11.9
• In each of your cells (except gametes and red blood cells), you have
23 pairs of chromosomes (DNA), for a total of 46
chromosomes(DNA).
• Copying DNA accurately is similar to communicating a message. The
DNA molecule itself serves as a template for its own replication.
– DNA can serve as its own template because its organization is
specific
– Double helix – A single DNA molecule is a double stranded
structure with the two strands twisting together in a spiral or
helical form
DNA Structure
• Nucleotides - a series of smaller molecules that make up the DNA
molecule
– Each nucleotide consists of a phosphate group, deoxyribose (a sugar), and a
nitrogen base
– The sugar and phosphate portions are the same in all nucleotides
– The nitrogen bases vary as like letters in a word . The order of these bases is
called the DNA sequence. These nitrogen bases along one strand of the DNA
convey information to certain parts of the cell.
•
•
•
•
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
• Nucleic acid – A long chain or strand of nucleotides
– RNA or ribonucleic acid
– DNA or deoxyribonucleic acid
Details of the DNA molecule Figure
11.11
• Each side of the DNA double helix, consists of nucleotides
alternating deoxyribose and phosphate groups.
– The deoxyribose and phosphate molecules form a covalent
bond with one another (strong bond).
– This creates the backbone of one side of the molecule and is
forms a nucleic acid
• The 4 different nitrogen bases on one strand of the DNA
interact with the nitrogen bases on the other strand of DNA
– Adenine forms a hydrogen bond (weak bond) with thymine and
guanine forms a hydrogen bond (weak bond) with cytosine.
– The sequence of nucleotides along the strand encodes the cell’s
genetic information
How does the DNA structure allow a DNA molecule to be copied
to make another identical molecule? Figure 11.13
• 1. Notice that the pairing that takes place between nitrogen bases
is specific in DNA strands. This bonding pattern is called
complementary base pairing.
– Adenine (large base) always pairs with Thymine (small base)
– Guanine (large base) always pairs with Cytosine (small base)
• 2. Replication begins when specific enzymes separate (unzip) the
two DNA strands. The location of this separation is called the
Replication Fork.
• 3. Enzymes read the sequence of nucleotides on one strand and
create a new complementary strand by adding one nucleotide at a
time to the new strand.
– The sequence of bases in the old strand determines the
sequence of bases in the new strand
– Each newly added base must complement the base in the old
strand with which it will pair.
– The two strands are copied in opposite directions.
– In eukaryotes this takes place in the nucleus of the cell.
• 4. There are two molecules of DNA where previously there had
been on. Each molecule contains one old strand and one new
complementary strand. Each resulting DNA is identical to the
original DNA and to each other.
• Pyrimidine
– single-ring nitrogenous base, found in DNA and RNA; either
cytosine, thymine, or uracil
• Purine
– double-ring nitrogenous base, found in DNA and RNA; either
adenine or guanine
Enzymes Involved in Replication and Mutation
• Helicase
– an enzyme that untwists the double helix at the replication
forks, separating the two parental strands and making them
available as template strands
• Replication fork
– A Y-shaped region on a replicating DNA molecule where new
strands are growing.
• DNA Polymerase III
– Adds new DNA nucleotides to a replicating DNA molecule
• DNA Polymerase II
– enzyme that proofreads the daughter strand of replicated DNA
and corrects any base pairing errors
• Ligase
– Glues together fragments of DNA
Mutations
• Point Mutation
• Insertion mutation is the adding of one or more base pairs to the
DNA molecule
• Deletion mutation is the removing of one or more base pairs to the
DNA molecule
• Substitution mutation is the replacing of a base pair with a different
base pair.
• Frameshift
– a deletion or insertion of base pairs which alters the reading of
the frame (3 at time), producing different amino acids