TEACHER COPY
Gene Expression Intro Assignment
PreAP Biology
gene expression - taking the information contained in the sequence of the nucleotides in the DNA
and using it to make protein
- 2 steps separated by the nuclear envelope in eukaryotes:
1) transcription - RNA synthesis
- occurs in the nucleus
2) translation - protein synthesis
- occurs at the ribosome in the cytoplasm
Review: nucleic acid, nucleotide, protein, amino acid
● Name the 2 nucleic acids: DNA & RNA
Complete the table below detailing the key differences between the 2 nucleic acids:
DNA
RNA
General structure/shape
double helix
single strand
Name of 5-C sugar
deoxyribose
ribose
thymine
uracil
Stores the genetic code in
Varies with type (there
its sequence of nucleotides
are at least 11 types)
Unique nitrogen base
Functions
1
●
●
●
●
●
DNA is always found in the nucleus. RNA molecules are made in the nucleus, but all forms of
RNA eventually leave the nucleus and enter the cytoplasm.
● What RNA nitrogen base bonds to adenine? uracil What RNA nitrogen base bonds to uracil?
adenine
What is the monomer of a nucleic acid? nucleotide
What are the 3 parts of a nucleotide? 5-C sugar, a phosphate group, and a nitrogen base
○ Draw a nucleotide. Label the 3 parts. Number the carbons of the sugar 1’ to 5’. Put a star by
the 3’ carbon.
Is a protein a polymer or a monomer? polymer
Amino acids are protein monomers
○ Draw the general structure of an amino acid. Circle the R
group. Label the amine (amino) group and the carboxylic
acid group.
***protein v polypeptide: We often use the terms protein and polypeptide
Interchangeably. However, they aren’t exactly the same thing. A polypeptide is a chain of
amino acids and often nothing more is meant than that. A protein is a functional
molecule that has multiple levels of structure - at least 3, sometimes 4.
A couple of terms used in genetics:
For the next few weeks and into the Spring semester, we
will be discussing phenotypes and genotypes of organisms. As a
reminder, phenotypes are the physical characteristics of organisms
- what you see. Genotypes are the versions of the genes (alleles)
contained in the nuclei of the cells of the organism. It is the
expression of the genotype that creates the phenotype. The
purpose of this unit is to explain to you how that expression of the
genotype happens.
Identify these as phenotypes (P) or genotypes (G):
P red
G aa
G LL
G Bb
P tall
P feathered
Also in genetics we will talk about mutations and
occasionally use the word mutant. This is not a derogatory term
when used correctly. If a plant originally produced red flowers,
but a change in the DNA leads to red pigment not being produced,
the flowers will be white. White flowering plants are therefore
“mutants” in this example. If there is an advantage to having
white flowers over red flowers (for example, if bees tend to
pollinate white flowers more often than red), the white flowers will become the most common in the
population of plants. We would then refer to the white flowering plant as “normal”. In genetics, “normal”
means the most common. Abnormal simple refers to the less common trait, which in this case would be red.
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Answer the following:
If a population of sheep is 35% horned and 65% not horned, which phenotype is the normal condition? Not
horned
If red eyes (RR) occur in 75% of fruit flies and sepia (Rr) in 5% of fruit flies (the other 20% are other colors),
what is the normal genotype for fruit fly eye color? RR
The Central Dogma:
What is the central dogma? In biological science, the phrase central dogma is used to describe the flow of
information from DNA to RNA to protein as unquestionable fact. We are going to look closely at this flow of
information, starting with the questions that lead to our understanding of the processes involved.
What causes diseases to “run in the family”?
Once Avery, and Hershey & Chase identified DNA (not protein) as the genetic material, the quest was
on to figure out HOW the order of the nucleotides resulted in the phenotypes we observe in organisms. The
answers came from observing the changes in phenotypes we refer to as genetic variation (when it’s neutral or
“good”) or as genetic abnormalities or genetic disorders (when it’s “bad”).
Let’s look at one of the well-documented examples of a “bad” phenotype. Dr. Archibald Gerrod (1902)
was the physician for a family in which “black urine disease” appeared generation after generation. Gerrod felt
that this was the result of an “inborn error in metabolism”. He was correct! The disorder, called alkaptonuria,
was investigated and it was determined that the problem is a missing enzyme in the pathway that allows a
component to pass out of the body intact. This component, alkapton, turns black when exposed to oxygen.
What would the normal condition be in humans, black urine or yellow urine? Yellow urine
What about neutral phenotype changes, often referred to as genetic variation? If you look at the
human population, you will see a variety of shades of color of eyes, hair, and skin. This is because we have
multiple genes that code for the product called melanin, a pigment responsible for coloration in humans
(except red heads) and many other mammals. The more genes that “work” and produce melanin, the darker
the eyes, hair, and skin of the person or organism; the fewer that work, the lighter the eyes, hair, and skin. In
this example, all of the genes working to produce melanin--resulting in brown skin, brown eyes, and brown
hair--is the normal condition.
Blue eyes is a phenotype / genotype (circle one). Is blue eyes normal, in the sense of genetics? No
From observations of disease like alkaptonuria and genetic variation like eye, hair or skin color, scientists
began conducting experiments to figure out exactly how these variations result.
One important discovery was made by Marshall Nirenberg (1961) and his team of researchers and
students. Following the Nobel-prize winning research of Beadle & Tatum that resulted in the formation of the
first version of the central dogma, one-gene → one-enzyme, Nirenberg set out to figure out the exact
relationship between the DNA sequence of nucleotides and the amino acid sequence of proteins. The result is
the mRNA codon chart we still use today. As our knowledge expanded so did our understanding of the central
dogma, which today has been refined to one gene → one polypeptide. (For more on this, google Beadle and
Tatum or Nirenberg).
The following terms are used to refer to the 3 nucleotide sequences in DNA, mRNA, and tRNA. You will
need to know these!
triplet - 3 DNA nucleotides (bases)
codon - 3 mRNA nucleotides (bases) → 1 amino acid
anticodon - 3 tRNA nucleotides (bases)
You will get context in the formal notes.
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Overview of Gene Expression
We have discussed how DNA replicates, how the nucleus divides in mitosis, and how the cell divides in cytokinesis.
We have looked at what a chromosome is, how it is inherited in sexual reproduction involving haploid gametes formed in
meiosis from 2 parents fusing to make a diploid zygote, and the basics of genetics (7th grade). In this unit we are going to
explore exactly how that sequence of nucleotides (gene) that codes for a protein results in a phenotype. How do we get
protein from a
sequence of DNA
nucleotides?
Section of condensed (metaphase) chromosome
Recall that DNA is in the nucleus. Any process involving DNA must therefore occur in the nucleus. Gene
expression, since it begins with a gene, a sequence of nucleotides in the DNA of a chromosome, begins in the nucleus. The
first part, transcription, can also be called RNA synthesis. Transcription involves an enzyme called RNA polymerase (use
your word parts--what does it do?) making an RNA copy of the DNA nucleotide sequence.
Once the RNA copy is made, it is modified a little, then it leaves the nucleus through a nuclear pore and enters the
cytoplasm. In the cytoplasm, the RNA binds to a ribosome which helps to A) position another kind of RNA so that B) the
amino acid attached to the other RNA can be chemically bonded to another amino acid. This second part, translation, can
also be called protein synthesis (recall that amino acids are the monomers of proteins).
At the end of the unit, we will
DNA have on the production of protein.
look at the effect mutations in the
https://media.hhmi.org/biointeractive/click/genetic-medicine-interactive/
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Gene Expression Notes
PreAP Biology
Background Information:
triplet - 3 DNA nucleotides; original source of the amino acid sequence
codon - 3 mRNA nucleotides; used to determine the correct amino acid coded for on the codon chart
anticodon - 3 tRNA nucleotides; determines which amino acid is attached to the end of the tRNA;
complementary to the mRNA codon
3 nucleotides = 1 codon = 1 amino acid
The diagram below shows 4 DNA triplets, 4 mRNA codons, and 4 amino acids
mRNA - transcript of the code from DNA taken from the nucleus to the ribosome in the cytoplasm
tRNA - translates the message by transferring (bringing) amino acids from the cytoplasm to the
ribosome based on the instructions
(codons) in the mRNA
rRNA - structural component of
ribosomes that binds mRNA and
tRNA together (most RNA in a cell is
rRNA)
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The process:
All RNA molecules are made from DNA. In these notes we are going to follow the synthesis
of a molecule of mRNA for simplicity.
TRANSCRIPTION
- RNA synthesis
- occurs in the nucleus
1) RNA polymerase binds to the DNA at a specific sequence of nucleotides (called the promoter-which includes the TATA box, but you don’t have to know these details)
2) RNA polymerase pries the 2 strands of DNA apart and
3) adds RNA nucleotides complementary to the DNA template strand in the 5’ to 3’ direction,
always adding to the 3’ end of the growing RNA transcript (note that the DNA template strand
is oriented 3’ to 5’)
4) the DNA double helix closes back up behind the RNA polymerase so the gene can be read
again by another RNA polymerase
RNA PROCESSING
- the mRNA made directly from the DNA template is called the primary transcript
- the primary transcript is not able to leave the nucleus and be translated into the
protein intended
- 3 modifications must be made:
1) a “special” guanine is added to the 5’ end = 5’ cap
2) several adenines are added to the 3’ end = poly-A tail
Both of these modifications (1 and 2):
- help the mRNA leave the nucleus
- protect the mRNA from enzymes that would chomp away at its ends
- help the mRNA attach to the ribosome
3) parts of the mRNA that don’t code for the protein that was signaled for are cut out (by a
protein/RNA complex called a spliceosome) and the parts that are needed are spliced together
= RNA splicing
intron - intervening sequence that does not code for the protein that is being made; part
that’s cut out
exon - expressed sequence that does code for the protein that is being made; part that is
needed and spliced together; the expressed part of the mRNA
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The product of these 3 steps is called the final transcript. It is now ready to leave the
nucleus through a nuclear pore into the cytoplasm.
THE RIBOSOME
Once in the cytoplasm, the mRNA will assemble into a translation complex with a ribosome.
The ribosome may be free in the cytoplasm or attached to the rough ER. The state of being “free”
or “attached” is not permanent; it changes with the needs of the cell.
The ribosome is composed of two subunits, one large and one small, that were made by the
knot of chromatin called the nucleolus.
The ribosome has important 3-dimensional structure. The small subunit has a place specific
for the binding of the mRNA molecule. The large subunit has 4 important features.
The first place that we will see in the description of translation below is referred to as the P
site. The P site is where the first tRNA attaches and the polypeptide is forming. The P site has a
tunnel that allows the amino acid chain (polypeptide) to leave the ribosome through the “top”
surface of the large subunit. On one side of the P site is the A site. The A site is where the next
tRNA carrying the next amino acid binds. On the other side of the P site is the E site. This is where
a tRNA (that has given up its amino acid) exits.
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TRANSLATION
- synthesis of a polypeptide from the sequence of nucleotides in an mRNA molecule in the
cytoplasm at the ribosome
- going from the language of nucleotides to the language of amino acids
1) initiation - small ribosomal subunit + mRNA + first tRNA followed by large ribosomal subunit
- as soon as these 4 components have assembled, the process of translation has started
(initiated)
- the first amino acid is always methionine (met); some codon charts just say “START”
- the mRNA codon for methionine is AUG; AUG is referred to as the start codon
- the tRNA anticodon found on the tRNA that brings methionine to the
mRNA/ribosomal complex is UAC
- the DNA triplet that coded for the AUG codon is TAC
2) elongation - amino acids are added one by one to the growing polypeptide chain
- the mRNA continues
moving through the
ribosome 1 codon (3
nucleotides), 5’ to
3’, at a time
- the first codon
(AUG) is closer to
the 5’ end of the
mRNA and the last
codon is closer to
the 3’ end
- the last codon does
not code for an
amino acid; it is
referred to as a
STOP codon; there
are 3 stop codons
(UAA, UAG, UGA)
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3) termination - when a stop codon is reached and no more amino acids are added; mRNAribosomal complex falls apart
- releasing the ribosomal subunits to be used again
- releasing the mRNA to be used again
- releasing the amino acid chain (polypeptide) into
- the cytoplasm where it will fold into its functional, 3D structure OR
- the rough ER where it will be modified and shipped to the Golgi (the Golgi will
finish the modifications and ship the finished protein to the location in the cell
that it is going to be needed, or to the membrane for insertion or secretion)
Important note about translation:
- just like DNA replication occurs at multiple “bubbles” along the length of the DNA molecule
so it goes faster and in both directions, multiple ribosomes join onto the same mRNA
molecule and translate it at the same time, following each other in a line, so that the amount
of protein produced is greater over the same amount of time; this structure of an mRNA
molecule with many ribosomes attached along its length is called a polyribosome
- and remember that in a prokaryote (bacteria), the DNA is not in the nucleus so as the mRNA is
being transcribed, translation can begin at the end already released from the DNA at the
same time
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Gene Expression - the whole process:
MUTATIONS
A mutation is a change in the DNA, specifically the sequence (order) of the DNA nucleotides.
Mutations can cause no change in a protein (silent), they can cause the protein to not function at
all, or cause the protein to not function the same way.
Terms you must know:
point mutation
silent mutation
sense mutation
nonsense mutation
missense mutation
•
substitution
insertion
deletion
frameshift
point mutation– a change in the genetic code that affects only one nucleotide
(base) in the DNA sequence
– can sometimes lead to a change in the amino acid sequence
– if a point mutation occurs that doesn’t change the amino acid
coded for, it is what is called a silent mutation.
A point mutation can lead to nonsense
•
nonsense mutation- results in a stop codon in the middle of the
amino acid (protein) and therefore it is completely nonfunctional
– stops the production of proteins, so no protein is made
A point mutation can lead to missense
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•
missense mutation- results in a complete protein, but with at least one amino acid different
– results in a different protein that is usually less functional than before
amino
There are 3 basic things that can occur in the nucleotide sequence to result in a sense or nonsense
mutation: substitutions, insertions, deletions
A substitution occurs when one base replaces another base.
An insertion is the addition of a base somewhere in the nucleotide sequence.
CTGGAG
CTGGTAG
A deletion is the loss of a nucleotide in the DNA sequence.
frameshift- when the mRNA nucleotides “shift” or move the whole
sequence over
• insertion and deletion can cause a frameshift
– a frameshift can create missense or nonsense
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12
Guided Practice
Transcription:
RNA Structure
HOW IS RNA DIFFERENT FROM DNA?
DNA (Deoxyribonucleic Acid)
o Contains the sugar deoxyribose
o Adenine_ pairs with Thymine
o Cytosine_ pairs with Guanine
o Double__ stranded
RNA (Ribonucleic acid)
o Contains the sugar ribose
o Adenine_ pairs with URACIL
o Cytosine_ pairs with Guanine
o Single___ stranded
Is uracil a purine or a pyrimidine? __pyrimidine_ How do you know? It has to be the same as thymine
Let’s Practice:
DNA Replication (Review)
Strand 1 (old strand) Strand 2 (new strand)
A
T
G
C
C
A
A
T
_T_
_A_
_C_
_G_
_G_
_T_
_T_
_A_
RNA Synthesis (Transcription) (Section 8.4)
DNA Template Strand
RNA Sequence
A
T
G
C
C
A
A
T
_U_
_A_
_C_
_G_
_G_
_U_
_U_
_A_
Compare Strand 2 (the new strand you wrote the sequence for) in DNA Replication to the RNA sequence that
resulted from RNA synthesis. What do you notice? The strands are identical except that in the RNA
sequence resulting from transcription have uracil (U) instead of thymine (T)
Where in the cell does RNA synthesis occur? The nucleus
What is RNA synthesis actually called? Transcription
What are the three types of RNA? __messenger__ RNA or __mRNA__, __transfer __RNA or _tRNA_, and
_ribosomal_ RNA or _rRNA_
Fill in the blanks to describe the function of each of the 3 types of RNA.
Messenger RNA (mRNA) transcribes the code from _DNA_ and takes it from the _nucleus_ into the
_cytoplasm_ at the ribosome.
Transfer RNA (tRNA) translates the message by transferring _amino_ _ acids _ from the cytoplasm to the
ribosomes based on the instructions in the mRNA.
Ribosomal RNA (rRNA) is a structural component of _ribosomes_ that binds mRNA and tRNA together
(most RNA in a cell is rRNA).
RNA contains the message from the DNA that is needed by the cell to make proteins.
What happens to the DNA molecule that was unzipped so that RNA synthesis (transcription) could occur?
The double helix closed back up and it is still in the nucleus for use by the cell
What are the 3 modifications made to mRNA before it can leave the nucleus?
● A 5’ guanasine cap is added to one end.
● A poly-A tail is added to the other end.
● The parts of the RNA that don’t code for amino acids, the introns , are cut out and the parts that do
code for amino acids, the exons , are spliced together.
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Where does the RNA molecule that was just made go now? Into the cytoplasm
How does it get out of the nucleus? Through a nuclear pore
Transcription: (Section 8.4)
● Transcription is when a molecule of _DNA_ untwists and one strand is read by RNA polymerase to make
a molecule of _RNA_. It occurs in the _nucleus_.
•
Nucleotides pair with one strand of DNA, and the enzyme RNA polymerase binds nucleotides together
Let’s “practice” transcription again.
T A C G C T A G T C C G T C
DNA
3’
mRNA
5’ A
5’
U G C G A U C A G G C A G 3’
Translation: (Section 8.5)
Translation is when an mRNA molecule is “read” in the cytoplasm at a ribosome, and tRNA molecules bring amino
acids in the order indicated by the nucleotide sequence of the mRNA molecule to be hooked together into a
polypeptide (protein).
Here is a diagram showing translation.
Write the number of the structure indicated in
the diagram next to the correct name.
amino acids
anticodon
mRNA
polypeptide
ribosome
1
1_
4
3
2
5
2
5
4
3
Let’s practice translating a message. Start by transcribing the DNA sequence given into a molecule of mRNA.
Divide the mRNA into codons by drawing a line between every 3 nucleotides in the mRNA code. Then, write the
anticodons that would be found on the corresponding tRNA molecule. Finally, use the codon chart to determine
which amino acid is coded for by the sequence in the mRNA.
DNA Sequence:
T A C G G G T T C A A C T T G A C T
mRNA Sequence:
(codons)
A U G C C C A A G U U G
A A C U G A
U A C G G G U U C A A C
U U G A C U
tRNA Sequence:
(anticodons)
Amino Acid Sequence:Start/met
pro
lys
leu
asp
stop
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How many codons did you write? __6_ How many anticodons did you write? _ 6_
How many amino acids were coded for? (HINT: Stop is NOT an amino acid) _ 5_
Recall the cell cycle. During which part of the cell cycle will most proteins be made and do the
job they were made for? G1 of interphase
Overview of Gene Expression:
What are the structures labeled 1 & 2?
1. Nucleus or nuclear envelope &
2. cytoplasm
1
This diagram shows only 1 ribosome
translating the message from the mRNA into
the polypeptide (amino acid sequence). Is this
accurate? No Explain. Hundreds of
ribosomes translate the mRNA at the
same time so that there is time for the
process to finish
When there is an error in the DNA (a
mutation), what can happen to the protein?
It can be nonfunctional, could stay the
same, not made at all, could be different
If you inherited a trait from one of your
parents, where is this trait coded for in you?
In my DNA (genes)
If red flowers are RR or Rr and white flowers
are rr, explain why these two alleles are
EXPRESSED differently. The R allele codes
for a protein that is a red pigment while
the r allele does not code for the
synthesis of any molecule
2
In the cell unit, you learned that proteins are modified and shipped around or even out of the cell in vesicles.
Which two organelles participated in the modifying of the proteins? Rough ER & Golgi
Based on what you know, explain why one gene → one enzyme is not as accurate as one gene → one
polypeptide. Because each gene codes for one amino acid chain which will not necessarily be an
enzyme (other types of proteins!), and protein implies that the molecule is functional, which may
not be true if quaternary structure (involving more than one polypeptide chain) is required.
Therefore a single amino acid chain/polypeptide is what is usually coded for, not an enzyme.
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On the lines in this box, write WHERE in the cell each process occurs. Be specific.
_____________
_nucleus______
________
________________
cytoplasm at the
ribosome_________
_______
SECRET MESSAGE PRACTICE
Use the mRNA codon chart to answer the following.
What is every student’s favorite animal?
DNA
B TCC O TTA ACA O
mRNA
B AGG O AAU UGU O
tRNA
B UCC O UUA ACA O
Amino acids
B arg
Amino acid symbols
B R O N C O
Answer
bronco
O asn
cys
O
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Mutations:
Directions: Using the original DNA strand below, determine the mRNA sequence and the
amino acids it codes for. Then, compare the original DNA/mRNA/amino acids to the mutated
DNA/mRNA/amino acids below. Determine the type of mutation (substitution, insertion,
deletion)
original DNA:
mRNA:
TAC AAA ATA GCA ACT
_ AUG
UUU UAU CGU UGA___
amino acid: __Start (Met) Phe Tyr Arg
Stop_______________________
T
inserted
mutated DNA:
mRNA: ___
Clue: one extra
nucleotide at end
TAC AAA ATT AGC AAC
T
AUG UUU UAA UCG UUG A__
amino acid: _____Start (met) Phe
STOP_______________________
type of mutation: __insertion__________ &
A replaced with T
2. mutated DNA:
TAC TAA ATA GCA
mRNA:__________________ AUG AUU UAU CGU
UGA_____________________________
amino acid: _______Start (met) Ile Tyr Arg
missing C
3. Mutated DNA:
TAC AAA ATA GAA
Clue: NOT 3
mRNA: ___________________AUG UUU UAU CUU GA__
amino acid: ________Start (met) Ile Ile
Val______________________
type of mutation: __deletion_____ & __nonsense_(NO stop
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Use your notes and the mRNA codon chart provided to determine the amino acid sequence.
DNA:
TAC
-
GAA
-
CCG
-
TCA
- TCG
- ACT
mRNA:
_AUG__ - _CUU__ - _GGC__ - _AGU_ - _AGC__ - _UGA__
tRNA:
_UAC_ - _GAA__ - _CCG__ - _UCA__ - _UCG__ - _ACU__
Amino Acid Sequence:
__met (start)_ - _Leu_____- _Gly__- _Ser___- _Ser____- ___STOP_
How many codons are in the above amino acid sequence? __6_____________
DNA:
TAC
-
CAT
-
TTG - GGA
- AGC
- ACT
mRNA:
_AUG_ - _GUA__ - _AAC__ - _CCU__ - _UCG_ - _UGA_
tRNA:
_UAC__ - _CAU__ - _UUG_ - _GGA__ - _AGC_ - _ACU_
Amino Acid Sequence:
_met (start)___ - _Val____- _Asn___- _Pro___- __Ser____- _STOP__
DNA:
_TAC_
- _CGC__ - _CCC__ - _AGC_ - _TCG__ - _ACT_
mRNA:
AUG
-
tRNA:
_UAC_ - _CGC__ - _CCC__ - _AGC_ - UCG__ - _ACU__
GCG
-
GGG -
UCG
- AGC
- UGA
Amino Acid Sequence:
_____start (met)___ - _Ala______- _Gly____- _Ser___- _Ser___- _STOP_______
DNA:
TAC___ - _CAC__ - __TTA__ - _AGG__ - AAA__ - _ACT___
mRNA:
_AUG___ - GUG___ - _AAU__ - _UCC__ - UUU__ - _UGA__
tRNA:
UAC
-
CAC
-
UUA
-
AGG
- AAA
- ACU
Amino Acid Sequence:
_______Start (met)_ - _Val_____- __Asp___- _Ser____- _Phe___- _STOP___
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Transcription - adapted from Biozone
19
Translation - adapted from Biozone
20
DNA DECODER TABLES
Table 1. mRNA codons
Second base in codon
U
C
A
G
U
UUU – phe
UUC – phe
UUA – leu
UUG – leu
UC* - ser
UAU – tyr
UAC – tyr
UAA – stop
UAG – stop
UGU - cys
UGC - cys
UGA - stop
UGG – trp
C
CU* - leu
CC* - pro
CAU – his
CAC - his
CAA - gln
CAG – gln
CG* - arg
A
AUU – ile
AUC – ile
AUA – ile
AUG – met
(start)
AC* - thr
AAU – asn
AAC – asn
AAA – lys
AAG – lys
AGU - ser
AGC - ser
AGA - arg
AGG - arg
G
GU* - val
GC* - ala
GAU – asp
GAC – asp
GAA – glu
GAG - glu
GG* - gly
First base in codon
*means that the last letter could be A, U, G, or C
Table 2. Amino Acids & their Symbols
AMINO ACID
Alanine
Arginine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
3-LETTER SYMBOL
ala
arg
asn
asp
cys
glu
gln
gly
his
ile
leu
lys
met
phe
pro
ser
thr
trp
tyr
val
1-LETTER SYMBOL
A
R
N
D
C
E
Q
G
H
I
L
K
M
F
P
S
T
W
Y
V
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Gene Expression Is A Regulated Process
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Gene expression, like all normal cell processes, is highly regulated. (Section 8.6 of your text) Genes are not
just the sequences of nucleotides that are transcribed; they also include regulatory sequences, such as the
promoter mentioned earlier, that help facilitate the process. The regulation in a prokaryotic cell is very straightforward (p.238-9). Eukaryotic regulation is much more complex and has many implications on the life of
multicellular organisms.
An important application of the regulation of gene expression involves the formation of different types of
cells. In humans, all embryos start out as a single diploid cell (zygote). As development continues into a fetus,
infant, and through adulthood, 200 types of cells develop. Remember the TedEd video at the beginning of the DNA
replication notes? In it the speaker talked about the chromosomes being a library, where each gene is a book.
Different types of cells have all the same books, but which books they are allowed to read varies. The
messages on what books (genes) to read (express) come from cell signals, including hormones, neurotransmitters,
and growth factors. The example used in the TedEd was insulin. Insulin is a protein produced only by specific cells
in the pancreas. However, insulin is not continuously produced. The cells only produce insulin when they receive a
message that blood sugar levels are high and insulin is needed to bring them down. Further, every cell in your body
has the book (gene) explaining how to make insulin, but only the specific pancreas cells with the receptors for the
signals that say “make insulin” can check out the book and read it (express the gene for insulin).
Stem Cells
The 200 different types of cells are all generated from the single diploid cell (zygote) each human starts out as. In the embryo,
stem cells reproduce themselves creating a ball of cells that are undifferentiated. Embryonic stem cells are pluripotent - capable of
differentiating themselves into many different cell types. Adult humans also have stem cells, but they are not capable of differentiating
into as many different types of cells as embryonic stem cells (multipotent). An example of adult stem cells are the cells in the bone
marrow that give rise to a variety of different types of blood cells. Stem cells differentiate into different types of cells based on which
genes are expressed and which genes are not expressed. (Which books in the chromosome library they read and which ones they don’t.)
Homeobox Genes
Also during embryonic development, homeotic genes (mastery regulatory genes) common to all
organisms lay out the axes of the embryo (which end is the head/tail, which side is ventral/dorsal, etc). These
genes must be turned on and back off at precise times in development for the embryo to turn out normal.
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Many of these master regulatory genes are responsible for turning other genes on and off during development,
like the breakers in a breaker box.
Apoptosis
Apoptosis is a type of programmed cell death. In a mature organism, it is a way in which cells selfdestruct to avoid damaged, infected, or abnormal cells from dividing and spreading. The enzymes required for
this process must be regulated at the level of gene expression to avoid accidental destruction. In a developing
embryo or fetus, apoptosis is part of the normal development process. For example, all humans have webbed
fingers and toes during development. At a particular point, the genes for the enzymes that destroy the cells in
the webbing are turned on, resulting in most babies being born with separated fingers and toes.
Amoeba Sisters: Why RNA is Just as Cool as DNA
~4:43 minutes
1. DNA is very important. It codes for your ________________.
2. Without _______________, you actually couldn’t get that genetic message out to your cells so that they
can start producing proteins.
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3. Compare and contrast RNA with DNA by putting the a check in the correct box for each character:
Character
DNA Only
RNA Only
Both DNA &
RNA
Nucleic acid
Deoxyribonucleic acid
Ribonucleic acid
Ribose sugar
Deoxyribose sugar
Double stranded
Double helix
Contains A, C, G
Contains T
Contains U
Found in nucleus
Can travel out of the
nucleus
4. There are actually 3 types of RNA:
_____________________________ (mRNA) – its job is to carry a message based off of the DNA
_____________________________ (tRNA) – its job is to transfer the message
_____________________________ (rRNA) – it’s actually a component of the ribosome
Need more? Need to hear it again? Need it explained differently?
https://www.youtube.com/playlist?list=PLwL0Myd7Dk1F0iQPGrjehze3eDpco1eVz
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Make sure you complete your Reading Guide and your Study Guide!
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