Download DNA Replication Replication begins simultaneously on several

DNA Replication
Replication begins simultaneously on several
chromatin threads & continues until all DNA has
been replicated
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Steps in DNA Replication (cont.)
Steps in DNA Replication
1)
DNA helices unwind from the nucleosomes
2)
Helicase untwists the double helix into 2 complementary
nucleotide chains exposing the nitrogenous bases
3)
Each nucleotide strand serves as a template for building a
new complementary strand. Occurs at the replication fork
4)
The replisome uses RNA primers to begin DNA synthesis
of 2nd strand
5)
DNA polymerase III continues from the primer and
covalently adds complementary nucleotides to the
template
6)
DNA ligase splices the short segments together
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DNA Replication
Since DNA polymerase only works in one direction:
A continuous leading strand is synthesized
A discontinuous lagging strand is synthesized
DNA ligase splices together the short segments of the
discontinuous strand
Two new telomeres are also synthesized
This process is called semiconservative replication
7) After replication, histones associate w/ the DNA,
chromatin strands condense forming chromatids, and are
held together by the centromere until anaphase when they
are distributed to each daughter cell
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Cell Division
Mitosis
Essential for body growth and tissue repair
DNA from the mother cell
Mitosis – nuclear division
The phases of mitosis are:
Cytokinesis – division of the cytoplasm
Meiosis produces ova and sperm w/ only ½ of the
number of genes found in the body cells
PLAY
Figure 3.31
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Prophase
Metaphase
Anaphase
Telophase
2 daughter cells
Figure 3.32 in Text
Mitosis
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1
Early Prophase
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Late Prophase
Figure 3.32.2
Metaphase
Chromosomes cluster at the middle of the cell with
their centromeres aligned at the exact center, or
equator, of the cell
This arrangement of chromosomes along a plane
midway between the poles is called the metaphase
plate
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Anaphase
Centromeres of the chromosomes split
Motor proteins in kinetochores pull chromosomes
toward poles
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 3.32.3
Metaphase
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Figure 3.32.4
Anaphase
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Figure 3.32.5
2
Control of Cell Division
Telophase and Cytokinesis
Surface-to-volume ratio of cells (e.g. cells outgrow
their membranes…cost effective to divide rather
than synthesize more membrane
Chemical signals such as growth factors and
hormones
Contact inhibition
Cyclins and cyclin-dependent kinases (Cdks)
complexes
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New nuclear membrane is formed from the rough
ER
Nucleoli reappear
Generally cytokinesis completes cell division
Cytokinesis
Figure 3.32.6
Protein Synthesis
DNA serves as master blueprint for protein
synthesis
Genes are segments of DNA carrying instructions
for a polypeptide chain (or a variety of RNAs)
4 nucleotides: A,C,G,T
Triplets of nucleotide bases form the genetic
library
Each triplet specifies coding for an amino acid or a
stop codon directing transcription to stop
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New sets of chromosomes extend into chromatin
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Telophase and Cytokinesis
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Cleavage furrow formed in late anaphase by
contractile ring
Cytoplasm is pinched into two parts after mitosis
ends
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Protein Synthesis
Many genes contain exons, regions encoding for a
polypeptide sequence, and
Introns, noncoding intervening sequences.
We are still not sure why introns exist
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3
From DNA to Protein
Roles of the Three Types of RNA
Nuclear
envelope
Transcription
Messenger RNA (mRNA) – carries the genetic
information from DNA in the nucleus to the
ribosomes in the cytoplasm
Transfer RNAs (tRNAs) – bound to amino acids
base pair with the codons of mRNA at the
ribosome to begin the process of protein synthesis
Ribosomal RNA (rRNA) – a structural component
of ribosomes
DNA
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Polypeptide
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Figure 3.33
Genetic Code
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Transcription
Rules by which base sequences of a gene are
translated into an amino acid sequence.
2 Major steps: Transcription
Translation
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Transfer of information from the sense strand of
DNA to RNA
Transcription factor
Loosens histones from DNA in the area to be
transcribed
Binds to promoter, a DNA sequence specifying the
start site of RNA synthesis
Mediates the binding of RNA polymerase to
promoter
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Transcription: RNA Polymerase
Coding
strand
Termination signal
Promoter
Template
strand
An enzyme that oversees the synthesis of RNA
Unwinds the DNA template
Adds complementary ribonucleoside triphosphates
on the DNA template
Transcription unit
RNA polymerase
bound to promoter
RNA
nucleotides
mRNA
Joins these RNA nucleotides together
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
nucleotides
RNA
polymerase
mRNA synthesis begins
RNA polymerase moves down DNA;
mRNA elongates
mRNA synthesis is terminated
DNA
Encodes a termination signal to stop transcription
(a)
mRNA transcript
Coding strand
RNA polymerase
Unwinding
of DNA
Rewinding of DNA
Template strand
RNA
nucleotides
mRNA
RNA-DNA
hybrid region
(b)
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Figure 3.34
4
Transcription
Codon: 3 base sequence on mRNA corresponding
to a specific amino acid
4 nucleotides (A,C,G,U) in RNA so there are:
43 = 64 codons
3 are stop codons (termination of the polypeptide
chain)
61 code for amino acids
There are only 20 amino acids, so more than 1
codon codes for a specific amino acid
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Transcription
Pre mRNA contains introns and exons
Pre mRNA is processed whereby the introns are
spliced out and the exons are spliced together.
This is done by the splicesome
mRNA complex proteins then associate w/ the
processed mature mRNA and guide its export from
the nucleu
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Genetic Code
RNA codons code
for amino acids
according to a
genetic code
Figure 3.35
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Translation
Nucleic acid sequences are “translated” into amino acid
sequences (polypeptides)
Occurs in the cytoplasm and involves mRNA, tRNA,
rRNA
A leader sequence on mRNA attaches to the small subunit
of the ribosome by base pairing to rRNA (ribosomal RNA)
tRNA (transfer RNA)loads a single amino acid, migrates to
the ribosome, and maneuvers the amino acid into position
as specified by the mRNA
tRNA has 2 active sites: 1) binding of amino acid at one
end, and 2) a 3-base complementary to the mRNA codon
(anticodon) calling for the amino acid carried by the
particular tRNA
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Translation
Nucleus
Nuclear membrane
RNA polymerase
Nuclear pore
Anticodons form hydrogen bonds w/
complementary codons (base pairing)
mRNA
Template strand
of DNA
tRNA is the link (translator) between nucleic acids
and amino acids
1
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
tRNA
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
There are 45 different tRNAs each capable of
binding to a specific amino acid
Codon 15 Codon 16 Codon 17
Direction of
ribosome advance
Portion of mRNA
already translated
tRNA “head”
bearing
anticodon
Large
ribosomal
subunit
2
4
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Amino acids
Released mRNA
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.
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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.
Figure 3.36
5
Information Transfer from DNA to RNA
DNA triplets are transcribed into mRNA codons
by RNA polymerase
Codons base pair with tRNA anticodons at the
ribosomes
Amino acids are peptide bonded at the ribosomes
to form polypeptide chains
Start and stop codons are used in initiating and
ending translation
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Information Transfer from DNA to RNA
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Figure 3.38
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Home Games
Tuesday Volleyball vs. WPU @ 7 pm and
Field Hockey vs. MSU @ 7:30 pm
Wednesday
Saturday Men’s Soccer vs. RutgersCamden @ 7pm and Football vs. MSC
@ 1pm
Men’s Soccer vs.
Rutgers-Newark @ 7:30 pm
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