2/6/2012
Chapter 4
Genetics and Cellular Function
Discovery of the Double Helix
• By 1900: components of DNA were known
– sugar, phosphate and bases
• DNA and RNA – the nucleic acids
• Genes and their action
• By 1953: x ray diffraction determined geometry of DNA
molecule
• DNA to Proteins
• 1962 Nobel Prize awarded to: Watson, Crick and Wilkins
– Rosalind Franklin died of cancer at 37.
4-1
4-2
DNA Structure
DNA Molecular Structure
• DNA – threadlike molecule with
uniform diameter, but varied length
• Double helix
(a)
A
T
G
C
A
T
A
T
G
C
A
T
C
G
T
A
– Composed of Nucleotides
1. phosphate group
2. deoxyribose sugar
3. nitrogenous base
T
C
G
N
• DNA and other nucleic acids are
polymers of nucleotides
HC
N
H
C
C
C
N
CH
N
O
• Each nucleotide consists of
HO
– one sugar - deoxyribose
– one phosphate group
– one nitrogenous base
G
C
(b)
Adenine
NH2
– How many in most human cells?
Nucleotides: phosphate
group, sugar, and
nitrogenous base
P
O
O
CH2
OH
H H
H H
OH
Phosphate
H
Deoxyribose
T
A
G
Sugar–phosphate
backbone
C
Hydrogen
bond
Sugar–phosphate
backbone
4-3
(c)
Nitrogenous Bases of DNA
Complementary Base Pairing
Purines
O
• There are only four
NH2
N
C
C
N
• Purines - double ring
– Adenine (A)
– Guanine (G)
C
H
N
C
CH
C
HN
C
NH
C
N
• Nitrogenous bases united by
hydrogen bonds
CH
NH
C
N
Adenine (A)
Guanine (G)
Pyrimidines
H
C
NH2
• Pyrimidines - single ring
– Cytosine (C)
– Thymine (T)
CH3
C
HC
C
N
N
H
O
C
HC
C
O
Cytosine (C)
–
–
C
O
Thymine (T)
O
HN
C
• DNA bases - ATCG
O
(b)
C
N
H
CH
CH
Uracil (U)
Figure 4.1b
T
A–T
C–G
• Law of Complementary Base
Pairing
4-5
– one strand determines base
sequence of other
C
G
• DNA base pairing
NH
N
H
T
– a purine on one backbone with a
pyrimidine on the other
– A – T two hydrogen bonds
– C – G three hydrogen bonds
NH2
G
C
A
G
C
Hydrogen
bond
Sugar–phosphate
backbone
Sugar–phosphate
backbone
Figure 4.2 partial….
4-6
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Chromatin and Chromosomes
DNA Function
• chromatin – thread of DNA material with
proteins
– occurs as 46 long filaments called
chromosomes
– in nondividing cells, chromatin is so
slender it cannot be seen
– histones – cluster of eight proteins
– nucleosome consists of :
• Histones with DNA around them
• Gene – segment of DNA that codes for a specific protein
• Genome - all the genes of one person (or species)
– humans have ~ 35,000
• 2% of total DNA
• other 98% is non-coding DNA
– plays role in chromosome structure
– regulation of gene activity
– no function at all – “junk” DNA
2 nm
1
DNA double
helix
11 nm
2
DNA winds
around core
particles to form
nucleosomes
11 nm in
diameter
Histone
Linker DNA
Nucleosome
30 nm 3
300 nm 4
In dividing cells only
700 nm 5
Chromatids
Nucleosomes
fold accordionlike into zigzag
fiber
30 nm fiber is
thrown into
irregular loops
to form a fiber
In dividing
cells, looped
chromatin coils
further into a
700 nm fiber to
form each
chromatid
Centromere
700 nm 6
Chromosome
at the midpoint
(metaphase) of
cell division
Figure 4.4b
4-7
4-8
What is a Gene?
Cells Preparing to Divide
• Previous definition - gene - a segment of DNA that carries
the code for a particular protein???
• exact copies are made of all DNA
(DNA replication)
– Body has millions of proteins but only 35,000 genes?
– Small % of genes produce only RNA molecules
– Some segments of DNA belong to 2 different genes
• each chromosome consists of two
parallel sister chromatids
– joined at centromere
– kinetochore – proteins
Kinetochore
• During cell division
- sister chromatids pull apart
- each new cell gets a chromatid
Centromere
• Amino acid sequence of a protein is determined by the
nucleotide sequence in the DNA
Sister
chromatids
(a)
Figure 4.5a
4-9
4-10
Human Genome
Human Genome
– Homo sapiens has only about 35,000 genes
• Genome – all the DNA in one 23-chromosome set
– genes generate millions of different proteins
• single gene can code for many different proteins
– 3.1 billion nucleotide pairs in human genome
• 46 human chromosomes comes in two sets of 23
chromosomes
– one set of 23 chromosomes came form each parent
– each pair of chromosomes (homologous chromosomes) has same
genes but different versions (alleles) exist
• Human Genome Project (1990-2003) identified
nitrogenous base sequences of 99% of the human genome
– genes average about 3,000 bases long
• range up to 2.4 million bases
– all humans are at least 99.99% genetically identical
• 0.01% variations that we can differ from one another in
more than 3 million base pairs
– some chromosomes are gene-rich and some gene-poor
– known locations for >1,400 disease-producing mutations
4-11
4-12
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Genetic Code
RNA: Structure and Function
• Millions of different proteins, all made from 20 amino acids,
– encoded by genes made of 4 nucleotides (A,T,C,G)
• Genetic code – Arrangement of nucleotides that code for
amino acid sequence of proteins
i.e., nucleotides arrangement determines amino acid arrangement
• Base triplet – a sequence of 3 DNA nucleotides that codes
for 1 amino acid
– codon - the 3 base sequence in mRNA
•
RNA
– Much smaller than DNA
- a single nucleotide chain
- Ribose sugar (not deoxyribose)
- No thymine nitrogenous base (replaced by Uracil)
3 types of RNA
1. messenger RNA (mRNA) over 10,000 bases
2. ribosomal RNA (rRNA) 3.
3. transfer RNA (tRNA) 70 - 90 bases
• Function
– interprets code in DNA
– uses those instructions for protein synthesis
4-13
Overview of Protein Synthesis
• all body cells contain identical genes (except sex cells and
some immune cells)
4-14
From DNA to Protein
Nuclear
envelope
• different genes are activated in different cells
Transcription
• Once activated a gene
– Makes messenger RNA (mRNA) – a mirror-image copy of the
gene is made
DNA
Pre-mRNA
RNA Processing
• migrates from the nucleus to cytoplasm
• its code is read by the ribosomes
mRNA
– ribosomes – ribosomal RNA (rRNA) and enzymes
Ribosome
– transfer RNA (tRNA) – delivers amino acids to the ribosome
Translation
– ribosomes assemble amino acids in the order directed by codons
of mRNA
Polypeptide
4-15
Figure 3.33
Transcription: RNA Polymerase
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• The enzyme that oversees mRNA synthesis
• Starts at a promoter site (thanks to a transcription
factor)
• Breaks H bonds & Unwinds DNA
• Adds complementary nucleotides on DNA template
strand
– following Law of Complimentary Base Pairing
– Joins RNA nucleotides together to match DNA
coding strand
• Encodes a termination signal to stop transcription
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Coding
strand
Termination signal
Promoter
Template
strand
Transcription unit
In a process mediated by a transcription
factor, RNA polymerase binds to
promoter and unwinds 16–18 base
pairs of the DNA template strand
RNA
polymerase
Unwound DNA
RNA polymerase
bound to promoter
RNA
nucleotides
mRNA
RNA
nucleotides
RNA
polymerase
mRNA synthesis begins
RNA polymerase moves down DNA;
mRNA elongates
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mRNA synthesis is terminated
DNA
(a)
mRNA transcript
Coding strand
RNA polymerase
Unwinding
of DNA
Rewinding of DNA
Template strand
RNA
nucleotides
mRNA
RNA-DNA
hybrid region
(b)
20
Figure 3.34
Fixing pre-mRNA
Splicing of mRNA
Gene (DNA)
• Pre-mRNA is much larger
than mRNA
– Contains non-coding
regions - introns
– Coding regions - exons
– In nucleus, introns are
removed and exons
spliced together to
produce final mRNA
1 Transcription
Intron
Pre-mRN A
A
B
C
D
Exon
E
F
Figure 4.6
2 Splicing
mRN A 1
A
C
mRN A 2
D
B
D
mRN A 3
E
A
E
F
3 Translation
Protein 1
•
•
Protein 2
Protein 3
One gene can code for more than one protein
Exons can be spliced together into a variety of different mRNAs.
4-22
3-38
Nucleus
From DNA to Protein
Nuclear membrane
RNA polymerase
Nuclear pore
Nuclear
envelope
mRNA
Template strand
of DNA
Transcription
Released mRNA
DNA
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Polypeptide
Figure 3.33
Figure 3.36
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Nucleus
Nuclear membrane
Nucleus
Nuclear membrane
RNA polymerase
Nuclear pore
RNA polymerase
Nuclear pore
mRNA
mRNA
Template strand
of DNA
Polysome: mRNA binding
to ribosome
Template strand
of DNA
Released mRNA
1
1
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
Small ribosomal
subunit
Codon 15
Codon 16 Codon 17
Amino acids
Released mRNA
tRNA
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
Codon 15
Direction of
ribosome advance
Portion of mRNA
already translated
Codon 16 Codon 17
Direction of
ribosome advance
Portion of mRNA
already translated
Large
ribosomal
subunit
Large
ribosomal
subunit
Energized by ATP,
the correct amino
acid is attached to
each species of tRNA
by aminoacyl-tRNA
synthetase enzyme.
Figure 3.36
Figure 3.36
Nucleus
Nuclear membrane
Nucleus
Nuclear membrane
RNA polymerase
Nuclear pore
RNA polymerase
Nuclear pore
mRNA
mRNA
Template strand
of DNA
Template strand
of DNA
Amino acids
Released mRNA
1
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
Codon 16 Codon 17
1
tRNA
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
Codon 15
Amino acids
Released mRNA
tRNA
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
Codon 15
Direction of
ribosome advance
Portion of mRNA
already translated
Codon 16 Codon 17
Direction of
ribosome advance
Portion of mRNA
already translated
tRNA “head”
bearing
anticodon
tRNA “head”
bearing
anticodon
Large
ribosomal
subunit
2
Incoming aminoacyltRNA hydrogen bonds
via its anticodon to
complementary mRNA
sequence (codon) at
the A site on the
ribosome.
Large
ribosomal
subunit
Energized by ATP,
the correct amino
acid is attached to
each species of tRNA
by aminoacyl-tRNA
synthetase enzyme.
Energized by ATP,
the correct amino
acid is attached to
each species of tRNA
by aminoacyl-tRNA
synthetase enzyme.
Incoming aminoacyltRNA hydrogen bonds
via its anticodon to
complementary mRNA
sequence (codon) at
the A site on the
ribosome.
2
As the ribosome
moves along the
mRNA, a new amino
acid is added to the
growing protein chain
and the tRNA in the A
site is translocated
to the P site.
3
Figure 3.36
Figure 3.36
Translation of mRNA
Nucleus
Nuclear membrane
RNA polymerase
Nuclear pore
Cytosol
8
mRNA
Nucleus
DNA
Template strand
of DNA
Ribosomal subunits
rejoin to repeat the
process with the same
or another mRNA.
Amino acids
7
5
Released mRNA
mRNA
1
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
Codon 15
Codon 16 Codon 17
4
tRNA
1
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
mRNA leaves
the nucleus.
2 Ribosome
binds mRNA.
Direction of
ribosome advance
The preceding tRNA hands off
the growing protein to the new
tRNA, and the ribosome links the
new amino acid to the protein.
tRNA anticodon binds
to complementary
mRNA codon.
GU
Protein
A
CGU
C A U GC
3
Portion of mRNA
already translated
tRNA “head”
bearing
anticodon
Large
ribosomal
subunit
Once its amino acid is
released, tRNA is
ratcheted to the E site
and then released to
reenter the cytoplasmic
pool, ready to be
recharged with a new
amino acid.
3
As the ribosome
moves along the
mRNA, a new amino
acid is added to the
growing protein chain
and the tRNA in the A
site is translocated
to the P site.
ADP
tRNA
2
4
Incoming aminoacyltRNA hydrogen bonds
via its anticodon to
complementary mRNA
sequence (codon) at
the A site on the
ribosome.
Energized by ATP,
the correct amino
acid is attached to
each species of tRNA
by aminoacyl-tRNA
synthetase enzyme.
A
tRNA binds an
amino acid; binding
consumes 1 ATP.
ATP
6
+
After translating the
entire mRNA, the
ribosome dissociates
into its two subunits.
Pi
Free tRNA
tRNA is released from
the ribosome and is
available to pick up a
new amino acid and
repeat the process.
Free amino acids
4-30
Figure 3.36
5
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31
Protein Synthesis
Information Transfer from DNA to RNA
Template strand
Figure 3.38
Review of Peptide Formation
3-43
Genetic Code:
• RNA codons code for
amino acids
according to a
genetic code
• Remember there are
20 amino acids
1 DNA double helix
2 triplets on the
template strand of DNA
3 Corresponding codons of
mRNA transcribed from the
DNA triplets
4 The anticodons of tRNA that
bind to the mRNA codons
5 The amino acids carried by
those six tRNA molecules
6 The amino acids linked into a
peptide chain
4-35
Figure 3.35
6
2/6/2012
Protein Processing and Secretion
• Proteins to be secreted are made in ribosomes of rough ER
– Contain a leader sequence of 30+ hydrophobic amino
acids
Inside ER leader sequence is removed; protein is
modified
• protein synthesis is not finished when the amino acid
sequence (primary structure) has been assembled.
• to be functional it must coil or folded into precise
secondary and tertiary structure
• Chaperone proteins (stress proteins or heat-shock
proteins)
4-37
3-46
Protein Packaging and Secretion
Mechanism of Gene Activation
Prolactin
Prolactin
receptors
1
1
Exocytosis
Protein formed by
ribosomes on rough ER.
ATP
2 Protein packaged into transport
vesicle, which buds from ER.
Nucleus
Casein
7
3 Transport vesicles fuse into clusters that
unload protein into Golgi complex.
2
4 Golgi complex modifies
protein structure.
Secretory
vesicles
AD P
+
Pi
6
Golgi
complex
5 Golgi vesicle containing
finished protein is formed.
Regulatory
protein
(transcription
activator)
6 Secretory vesicles
release protein by
exocytosis.
Ribosomes
Clathrin-coated
transport vesicle
Golgi
complex
Rough
Endoplasmic
reticulum
5
3
Rough ER
4
Lysosome
Casein
gene
Figure 4.11
mRNA
for casein
RNA
polymerase
4-39
4-40
Synthesizing Compounds Other
Than Proteins
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• Cells synthesize glycogen, fat, steroids, phospholipids,
pigments, and other compounds
–
–
–
–
no genes for these
synthesis under indirect genetic control
produced by enzymatic reactions
enzymes are proteins encoded by genes
• example – testosterone production
–
–
–
–
a steroid
a cell of the testes takes in cholesterol
enzymatically converts it to testosterone
only occurs when genes for enzyme are activated
4-42
7