DNA

1
DNA RNA

• Chapter 12

2
The Structure of DNA

• DNA is a long macromolecule made up of units
  called nucleotides
• Each nucleotide is made up of 3 basic parts
• a 5-carbon sugar (deoxyribose)
• a phosphate group
• a nitrogenous base
• The function of DNA is to store and transmit
  genetic information

3
The Structure of DNA

• There are 4 kinds of nitrogenous bases in DNA
• The PURINES (larger, double rings)
• A Adenine
• G Guanine
• The PYRIMIDINES (smaller, single rings)
• C Cytosine
• T Thymine
• The backbone of a DNA chain is formed by sugar
  and phosphate groups of each nucleotide

4
The Double Helix

• The work of physicists Rosalind Franklin, James
  Watson, and Francis Crick led to the discovery of
  the shape of a DNA molecule
• In 1953, Watson Crick published a paper
  explaining that the structure of DNA was a double
  helix, in which 2 strands were wound around each
  other

5
The Double Helix

• A double helix looks like a twisted ladder or
  spiral staircase
• The 2 strands of the double helix are held
  together by hydrogen bonds between adenine and
  thymine and between guanine and cytosine
• A T always pair and C G always pair this is
  known as the principle of base-pairing

6
Structure of DNA
Section 12-1
Nucleotide
Hydrogen bonds
Sugar-phosphate backbone
Key Adenine (A) Thymine (T) Cytosine (C) Guanine
(G)
Go to Section
7
DNA Chromosomes

• Because they lack nuclei, the DNA of prokaryotic
  cells are located in the cytoplasm
• Most prokaryotes have a single circular DNA
  molecule that contains all of the cells genetic
  information
• Eukaryotic DNA is found in the cell nucleus in
  the form of many chromosomes
• The number of chromosomes will vary by species

8
Chromosome Structure

• Eukaryotic chromosomes contain both DNA and
  protein, tightly packed together to form a
  substance called chromatin
• Chromatin consists of DNA that is tightly coiled
  around proteins called histones
• Together, the DNA and histone molecules form a
  beadlike structure called a nucleosome

9
DNA Replication

• Each strand of the DNA double helix has all the
  information needed to reconstruct the other half
  by the mechanism of base pairing
• Because each strand can be used to make the other
  strand, the strands are said to be complementary

10
Duplicating DNA

• Before a cell divides, it duplicates its DNA in a
  copying process called replication
• During DNA replication, the DNA molecule
• Separates into 2 strands
• Produces 2 new complementary strands following
  the rules of base pairing
• Each strand of the double helix of DNA serves as
  a template, or model, for the new strand

11
How Replication Occurs

• DNA replication is carried out by a series of
  enzymes
• These enzymes unzip the DNA molecule by
  breaking the hydrogen bonds between base pairs
  and unwinding the 2 strands
• Each strand then serves as a template for the
  attachment of complementary bases
• Ex the strand with bases TACGTT produces a
  strand with complementary bases ATGCAA
• The result is 2 DNA strands that are identical,
  each one having one original strand and one new
  strand
• The principal enzyme involved in DNA replication
  is DNA polymerase
• DNA polymerase proofreads new DNA strands,
  helping to ensure that each molecule is a nearly
  perfect copy of the original DNA

12
DNA Replication
Section 12-2
Original strand
DNA polymerase
New strand
Growth
DNA polymerase
Growth
Replication fork
Replication fork
Nitrogenous bases
New strand
Original strand
Go to Section
13
Visual Overview of DNA Replication
14
The Structure of RNA

• RNA, like DNA, consists of a long chain of
  nucleotides
• There are 3 main differences between RNA DNA
• The sugar in RNA is ribose instead of deoxyribose
• RNA is generally single stranded instead of
  double stranded
• RNA contains uracil in place of thymine

15
Types of RNA

• In most cells, RNA molecules are involved in just
  one job protein synthesis
• The assembly of amino acids into proteins is
  controlled by RNA
• There are 3 main types of RNA
• Messenger RNA (mRNA)
• Ribosomal RNA (rRNA)
• Transfer RNA (tRNA)

16
Types of RNA

• Messenger RNA carry copies of instructions for
  the assembly of amino acids into proteins from
  DNA to the rest of the cell
• Ribosomal RNA make up the major parts of
  ribosomes
• Transfer RNA transfers amino acids to ribosomes
  during protein synthesis

17
Transcription Making RNA From DNA

• RNA molecules are produced by copying part of the
  nucleotide sequence of DNA into a complementary
  strand of RNA in a process called transcription
• Transcription requires an enzyme known as RNA
  polymerase
• During transcription, RNA polymerase binds to a
  promoter site on a strand of DNA and separates
  the DNA strand
• RNA polymerase then uses one strand of DNA as a
  template from which nucleotides are assembled
  into a strand of RNA

18
Transcription Making RNA From DNA
Adenine (DNA and RNA) Cystosine (DNA and
RNA) Guanine(DNA and RNA) Thymine (DNA
only) Uracil (RNA only)
RNApolymerase
DNA
RNA
19
RNA Editing

• Before RNA is functional, editing is often
  required
• Many times, large pieces are removed from the
  transcribed RNA molecule
• These large pieces that are spliced out are known
  as introns
• The remaining portions of RNA, called exons
  (expressed sequences), are then spliced back
  together to form the final RNA

20
RNA Concept Map
Section 12-3
RNA
can be
also called
which functions to
also called
also called
which functions to
which functions to
from
to
to make up
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21
The Genetic Code

• Proteins are made by joining amino acids into
  long chains called polypeptides.
• Each polypeptide contains a combination of any or
  all of the 20 different amino acids
• The properties of proteins are determined by the
  order in which different amino acids are joined
  together to produce polypeptides

22
The Genetic Code

• The language of mRNA instructions is called the
  genetic code.
• Recall that RNA contains 4 different bases A, U,
  C, G
• The genetic code is read 3 letters at a time so
  that each word of the coded message is three
  bases long
• Each three-letter word in mRNA is known as a
  codon
• A codon consists of 3 consecutive nucleotides
  that specify a sing amino acid that is to be
  added to the polypeptide

23
The Genetic Code

• Consider the following mRNA sequence
• UCGCACGGU
• The sequence would be read three bases at a time
  as
• UCG-CAG-GGU
• The condons represent the different amino acids
• UCG - CAG - GGU
• Serine-Histidine-Glycine

24
The Genetic Code
Section 12-3
To decode a codon, start a the middle of the
circle and move outward. Notice that there are 3
stop codons that do not code for any amino acid
instead they signify the end of a polypeptide.
Go to Section
25
Translation

• The decoding of an mRNA message into a
  polypeptide chain (protein) is known as
  translation.
• Translation takes place on ribosomes
• During translation, the cell uses information
  from mRNA to produce proteins

26
Translation
Section 12-3
Nucleus
Messenger RNA Messenger RNA is transcribed in
the nucleus using DNA as a template.
mRNA
Lysine
Phenylalanine
tRNA
Transfer RNA The mRNA then enters the cytoplasm
and attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has an
anticodon whose bases are complementary to a
codon on the mRNA strand. The ribosome positions
the start codon to attract its anticodon, which
is part of the tRNA that binds methionine. The
ribosome also binds the next codon and its
anticodon.
Methionine
Ribosome
Start codon
mRNA
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27
Translation (continued)
Section 12-3
Growing polypeptide chain
The Polypeptide Assembly Line The ribosome
joins the two amino acidsmethionine and
phenylalanineand breaks the bond between
methionine and its tRNA. The tRNA floats away,
allowing the ribosome to bind to another tRNA.
The ribosome moves along the mRNA, binding new
tRNA molecules and amino acids.
Ribosome
tRNA
Lysine
tRNA
mRNA
Completing the Polypeptide The process continues
until the ribosome reaches one of the three stop
codons. The result is a growing polypeptide
chain.
mRNA
Translation direction
Ribosome
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28
Mutations

• Mutations are changes in the DNA sequence that
  affect genetic information
• Gene mutations result from changes in a single
  gene
• Chromosomal mutations involve changes in whole
  chromosomes
• Mutations that affect one nucleotide are called
  point mutations
• Frameshift mutations are those that shift the
  reading frame of the genetic message by
  inserting or deleting a nucleotide

29
Gene Mutations Substitution, Insertion, and
Deletion
Section 12-4
Deletion
Substitution
Insertion
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30
Chromosomal Mutations
Section 12-4
Deletion
Duplication
Inversion
Translocation
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