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1. BIOLOGY: Today and Tomorrow, 4e
starr evers starr
Chapter 7
Gene Expression and Control

2. 7.1 Ricin and Your Ribosomes
 The ability to make proteins is critical to all life processes
 Seeds of the castor-oil plant contain the protein ricin, a deadly
poison that inactivates ribosomes that assemble proteins
 Ricin has been used by assassins, and is banned as a
weapon under the Geneva Protocol

3. Seeds of the castor-oil plant

4. 7.2 DNA, RNA, and Gene Expression
 A gene is a DNA sequence that encodes an RNA or protein
product in the sequence of its nucleotide bases (A, T, G, C)
 In transcription, enzymes use the gene’s DNA sequence as
a template to assemble a strand of messenger RNA (mRNA)
 In translation, the protein-building information in mRNA is
decoded into a sequence of amino acids
 The result is a polypeptide chain that folds into a protein

5. sugar–
phosphate
backbone
base pair
nucleotide
base
deoxyribonucleic acid
DNA
ribonucleic acid
RNA
DNA and RNA

6. Gene Expression
 Gene expression involves transcription (DNA to mRNA), and
translation (mRNA to protein)
 Gene expression
 Process by which the information in a gene becomes
converted to an RNA or protein product
 Proteins (enzymes) assemble other molecules and perform
many functions that keep the cell alive

7. 7.3 Transcription: DNA to RNA
 During transcription, a strand of DNA acts as a template upon
which a strand of RNA is assembled from nucleotides
 Base-pairing rules in DNA replication apply to RNA synthesis
in transcription, but RNA uses uracil in place of thymine
 The enzyme RNA polymerase, not DNA polymerase, adds
nucleotides to the end of a growing RNA strand

8. Base Pairing in Transcription

9. The Process of Transcription
 In transcription, RNA polymerase binds to a promoter in the
DNA near a gene
 Polymerase moves along the DNA, unwinding the DNA so it
can read the base sequence
 RNA polymerase links RNA nucleotides in the order
determined by the base sequence of the gene
 The new mRNA is a copy of the gene from which it was
transcribed

10. RNA
polymerase
gene
region
binding
site in
DNA
The enzyme RNA polymerase binds to a promoter in the DNA. The binding
positions the polymerase near a gene. Only one of the two strands of DNA will be
transcribed into RNA.
1
RNA polymerase binds to a promoter

11. RNA
DNA
winding up
DNA
unwinding
The polymerase begins to move along the gene and unwind the DNA. As it does, it links
RNA nucleotides in the order specified by the nucleotide sequence of the template DNA
strand. The DNA winds up again after the polymerase passes. The structure of the “opened”
DNA at the transcription site is called a transcription bubble, after its appearance.
2
RNA nucleotides are linked

12. direction of transcription
Zooming in on the transcription bubble, we can see that RNA polymerase
covalently bonds successive nucleotides into an RNA strand. The new strand is an RNA
copy of the gene.
3
RNA nucleotides are linked

13. Three Genes Being Transcribed
 Many polymerases transcribe a gene region at the same time
RNA molecules DNA molecule

14. RNA Modifications
 Eukaryotic cells modify their RNA before it leaves the nucleus
 Sequences that stay in the RNA are exons
 Introns are sequences removed during RNA processing
 Exons can be spliced together in different combinations, so
one gene may encode different proteins
 After splicing, a tail of 50 to 300 adenines (poly-A tail) is
added to the end of a new mRNA

15. gene
promoter exon intron exon intron exon
DNA
transcription
newly
transcribed
RNA
exon intron exon intron exon
exon exon exon
poly-A tail
finished mRNA
Post-transcriptional modification of RNA

16. ANIMATED FIGURE: Pre-mRNA transcript
processing

17. ANIMATED FIGURE: Gene transcription details

18. ANIMATED FIGURE: Negative control of the
lactose operon

19. 7.4 RNA Players in Translation
 Three types of RNA are involved in translation: mRNA, rRNA,
and tRNA
 mRNA produced by transcription carries protein-building
information from DNA to the other two types of RNA for
translation

20. mRNA and the Genetic Code
 Information in mRNA consists of sets of three nucleotides
(codons) that form “words” spelled with bases A, C, G, U
 Sixty-four codons, most of which specify amino acids,
constitute the genetic code
 The sequence of three nucleotides in a base triplet
determines which amino acid the codon specifies
 The order of codons in mRNA determines the order of amino
acids in the polypeptide that will be translated from it

21. Genetic Code
 Twenty amino acids are encoded by the sixty-four codons in
the genetic code
 Some amino acids are specified by more than one codon
 Other codons signal the beginning and end of a protein-
coding sequence
 Most organisms use the same code

22. The Genetic Code

23. a gene
region in DNA
transcription
codon codon codon
mRNA
translation
methionine
(met)
tyrosine
(tyr)
serine
(ser)
amino acid
sequence
Correspondence between
DNA, RNA, and proteins

24. rRNA and tRNA – the Translators
 Ribosomes consist of two subunits of rRNA and structural
proteins
 Ribosomes and transfer RNAs (tRNA) interact to translate an
mRNA into a polypeptide
 tRNA has two attachment sites
 An anticodon base-pairs with an mRNA codon
 An attachment site binds to an amino acid specified by the
codon

25. Ribosome Structure
large subunit small subunit intact ribosome
+ =

26. anticodon
A) Icon and model of the tRNA that carries the amino acid tryptophan. Each
tRNA’s anticodon is complementary to an mRNA codon. Each also carries the
amino acid specified by that codon.
tRNA for Tryptophan

27. B) During translation, tRNAs dock at an intact ribosome (for clarity, only the small subunit
is shown, in tan). Here, the anticodons of two tRNAs have base-paired
with complementary codons on an mRNA (red).
tRNAs dock at a ribosome

28. ANIMATED FIGURE: Structure of a ribosome

29. 7.5 Translating the Code: RNA to Protein
 Translation (second part of protein synthesis) occurs in the
cytoplasm of all cells:
 mRNA is transcribed in the nucleus
 In the cytoplasm a small ribosomal subunit binds to mRNA
 Initiator tRNA base-pairs with the first mRNA codon
 Large ribosomal subunit joins the small subunit
 Ribosome assembles a polypeptide chain
 Translation ends when the ribosome encounters a stop
codon

30. Translation in Eukaryotes
Transcription
ribosome
subunitsRNA transport
tRNA
1
Convergence of RNAs
mRNA
Translation
polypeptide
2
3
4

31. Ribosome assembles a polypeptide chain

32. Ribosome assembles a polypeptide chain

33. Ribosome assembles a polypeptide chain

34. ANIMATED FIGURE: Translation

35. 7.6 Mutated Genes and Their Products
 Mutations are permanent changes in the nucleotide sequence
of DNA, which may alter a gene product
 A mutation that changes a gene’s product may have harmful
effects
 Example: Mutations that affect the proteins in hemoglobin
reduce blood’s ability to carry oxygen

36. Types of Mutations
 Base-pair substitution
 Type of mutation in which a single base-pair changes
 Example: Sickle cell anemia
 Mutations that shift the reading frame of the mRNA codons:
 Deletion of one or more base pairs
 Insertion of one or more base pairs
 Example: Beta thalassemia

37. A) Hemoglobin, an oxygen-binding protein in red blood cells. This protein consists of four
polypeptides: two alpha globins (blue) and two beta globins (green). Each globin forms a
pocket that cradles a type of cofactor called a heme (red ). Oxygen gas binds to the iron
atom at the center of each heme.
Mutations in Hemoglobin

38. B) Part of the DNA (blue), mRNA (brown), and amino acid sequence (green) of human beta
globin. Numbers indicate the position of the nucleotide in the coding sequence of the
mRNA.
Mutations in Hemoglobin

39. C) A base-pair substitution replaces a thymine with an adenine. When the altered mRNA is
translated, valine replaces glutamic acid as the sixth amino acid of the polypeptide.
Hemoglobin with this form of beta globin is called HbS, or sickle hemoglobin.
Mutations in Hemoglobin

40. D) A deletion of one nucleotide causes the reading frame for the rest of the mRNA to shift.
The protein translated from this mRNA is too short and does not assemble correctly into
hemoglobin molecules. The result is beta thalassemia,
in which a person has an abnormally low amount of hemoglobin.
Mutations in Hemoglobin

41. E) An insertion of one nucleotide causes the reading frame for the rest of the mRNA to
shift. The protein translated from this mRNA is too short and does not assemble correctly
into hemoglobin molecules. As in D, the outcome is beta thalassemia.
Mutations in Hemoglobin

42. glutamic acid valine
A) A base-pair substitution results in the abnormal beta globin chain of sickle hemoglobin (HbS). The
sixth amino acid in such chains is valine, not glutamic acid. The difference causes HbS molecules to form
rod-shaped clumps that distort normally round blood cells into sickle shapes.
Sickle-Cell Anemia: A Base-Pair Substitution

43. sickled cell
normal cell
B) Left, the sickled cells clog small blood vessels, causing circulatory problems that result in damage to
many organs. Destruction of the cells by the body’s immune system results in anemia. Right, Tionne “T-
Boz” Watkins of the music group TLC is a celebrity spokesperson for the Sickle Cell Disease Association
of America. She was diagnosed with sickle-cell anemia as a child.
Sickle-Cell Anemia

44. What Causes Mutations?
 Most mutations result from unrepaired DNA polymerase
errors during DNA replication
 Some natural and synthetic chemicals cause mutations in
DNA (example: cigarette smoke)
 Insertion mutations may be caused by transposable
elements, which move within or between chromosomes

45. ANIMATED FIGURE: Base-pair substitution
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46. ANIMATION: Deletion
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47. ANIMATION: Frameshift mutation
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48. ANIMATED FIGURE: Sickle-cell anemia
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49. ANIMATED FIGURE: Controls of eukaryotic
gene expression
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50. ANIMATION: X-chromosome inactivation
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51. 7.7 Eukaryotic Gene Controls
 All cells in your body carry the same DNA
 Some genes are transcribed by all cells, but most cells are
specialized (differentiated) to use only certain genes
 Which genes are expressed at a given time depends on the
type of cell and conditions

52. Cell Differentiation
 Cells differentiate when they start expressing a unique subset
of their genes – controls over gene expression are the basis
of differentiation
 Differentiation
 The process by which cells become specialized
 Occurs as different cell lineages begin to express different
subsets of their genes

53. Controlling Gene Expression
 Controlling gene expression is critical for normal development
and function of a eukaryotic body
 All steps between transcription and delivery of gene product
are regulated
 Transcription factor
 Protein that influences transcription by binding to DNA

54. Master Genes
 Master gene
 Gene encoding a product that affects the expression of
many other genes
 Controls an intricate task such as eye formation
 Homeotic gene
 Type of master gene that controls formation of specific
body parts during development

55. Studying Homeotic Genes
 Researchers study the function of a homeotic gene by altering
its expression – by introducing a mutation or deleting it
entirely (gene knockout)
 Examples: antennapedia, dunce, tinman, groucho
 Many homeotic genes are interchangeable among species
 Example: eyeless gene in flies and PAX 6 gene in humans

56. A) A transcription factor—the
protein product (gold )
of an insect gene called
antennapedia attaches to a
promoter sequence in a fragment
of DNA. In cells of
a fly embryo, the binding starts a
cascade of cellular
events that results in the
formation of a leg.
Example of gene control

57. B) Antennapedia is a homeotic gene whose expression in embryonic tissues of the insect thorax causes
legs to form. A mutation that causes antennapedia to be expressed in the embryonic tissues of the head
causes legs to form there too (left). Compare the head of the normal fly on the right.
Example of gene control

58. Gene Knockout Experiment: Eyeless
A) A fruit fly with a mutation in
its eyeless gene develops
with no eyes.
B) Compare the large, round
eyes of a normal fruit fly.
C) Eyes form wherever the
eyeless gene is expressed in fly
embryos. Abnormal expression
of the eyeless gene in this fly
caused extra eyes to develop
on its head and also on its
wings.

59. PAX6 Gene Function
 In humans and many
other animals, the
PAX6 gene affects eye
formation
D) Humans, mice, squids, and other animals have a
gene called PAX6. In humans, PAX6 mutations
result in missing irises, a condition called aniridia
(left ). Compare a normal iris (right ). PAX6 is so
similar to eyeless that it triggers eye development
when expressed in fly embryos.

60. Sex Chromosome Genes
 In mammals, males have only one X chromosome – females
have two, but one is tightly condensed into a Barr body and
not expressed
 According to the theory of dosage compensation, X
chromosome inactivation equalizes expression of X
chromosome genes between the sexes

61. X Chromosome Inactivation
A) Barr bodies. The photo on the left shows the nucleus of five XX cells. Inactivated X chromosomes—
Barr bodies— appear as red spots. Compare the nucleus of two XY cells in the photo on the right

62. The Y Chromosome
 The human X chromosome carries 1,336 genes
 The human Y chromosome carries 307 genes, including
SRY— the master gene for male sex determination
 Triggers formation of testes
 Testosterone produced by testes controls formation of
male secondary traits
 Absence of SRY gene in females triggers development of
ovaries, female characteristics

63. SRY gene expressed no SRY present
penis
vaginal
opening
birth approaching
B) An early human embryo appears neither male nor female. SRY gene expression
determines whether male reproductive organs develop.
Development of
Human
Reproductive
Organs

64. Epigenetics
 Transcription is affected by chromosome structure
 Modifications that suppress gene expression:
 Adding a methyl group (CH3) to a histone protein
 Direct methylation of DNA nucleotides
 Once a particular nucleotide has become methylated, it
usually stays methylated in all of the cell’s descendants
 Environmental factors, including the chemicals in cigarette
smoke, add more methyl groups

65. Methyl group attached to a DNA nucleotide

66. Epigenetics
 Methylation of parental chromosomes is normally “reset” in
the first cell of the new individual
 All parental methyl groups are not removed, so some
methylations can be passed to future offspring
 Boys are affected by lifestyle of individuals in the father’s line;
girls, by individuals in the mother’s line
 Heritable changes in gene expression that are not due to
changes in underlying DNA sequence are epigenetic

67. An epigenetic effect
 Grandsons of boys who
endured a winter of famine
tend to live longer than
grandsons of boys who
overate at the same age

68. 7.8 Ricin and Your Ribosomes (revisited)
 Ricin is a ribosome-inactivating protein (RIP)
 Toxic RIPs, including ricin, have one polypeptide chain that
binds tightly to carbohydrates on plasma membranes
 Once inside the cell, a second polypeptide inactivates the
ribosomes, and the cell quickly dies
 Other RIPs include Shiga toxin (dysentery) and E. coli
O157:H7 (food poisoning)

69. Some RIPs
ricin Shiga toxin E. coli enterotoxin

70. Digging Into Data: Paternal Grandmother’s
Food Supply and Infant Mortality