Download Chapter 3_Part II

The Cell Continued
Golgi Apparatus
• Directs the trafficking of synthesized
proteins
• Functions in modification, concentration,
and packaging of proteins made in the
rough ER
• Transport vessels from the ER fuse with
the cis face of the Golgi apparatus
• Modifications involve the addition or removal of
sugars and phosphates
Golgi Apparatus
• Proteins then pass through the Golgi
apparatus to the trans face
• Secretory vesicles leave the trans face of
the Golgi stack and move to designated
parts of the cell
– Three paths can be take:
– Exocytosis
– Plasma membrane
– lysosome
Golgi Apparatus
Figure 3.20a
Role of the Golgi Apparatus
Rough ER
Cisterna
Proteins in cisterna
Phagosome
Membrane
Vesicle
Lysosomes containing acid
hydrolase enzymes
Vesicle incorporated
Pathway 3
into plasma membrane
Coatomer
coat
Golgi
apparatus
Pathway 2
Secretory vesicles
Pathway 1
Plasma membrane
Proteins
Secretion by exocytosis
Extracellular fluid
Figure 3.21
Lysosomes
• Spherical membranous bags containing
digestive enzymes
– Low pH in lysosome
• Abundant in phagocytes
• Digest ingested bacteria, viruses, and
toxins
• Degrade nonfunctional organelles
• Breakdown glycogen and release thyroid
hormone
Lysosomes
• Breakdown nonuseful tissue (during development,
termed autolysis) and organelles
(lysosomal rupture yields cell death via autolysis)
– Digest particles taken in by endocytosis
– Breakdown glycogen for energy
– Breakdown bone to release Ca2+
• Secretory lysosomes are found in white blood cells,
immune cells, and melanocytes
Endomembrane System
• System of organelles that function to:
– Produce, store, and export biological
molecules
– Degrade potentially harmful substances
• System includes:
– Nuclear envelope, smooth and rough ER,
lysosomes, vacuoles, transport vesicles, Golgi
apparatus, and the plasma membrane
Endomembrane System
Figure 3.23
Peroxisomes
• Membranous sacs containing oxidases
and catalases
• Detoxify harmful or toxic substances
• Neutralize dangerous free radicals
– Free radicals – highly reactive chemicals with
unpaired electrons (i.e., O2–)
– Oxidases + free radicals
H2O2 + catalase
H2O
Cytoskeleton
• The “skeleton” of the cell
• Consists of microtubules, microfilaments,
and intermediate filaments running
through the cytosol to give structure and to
support the organelles
Cytoskeleton
Figure 3.24a-b
Microtubules
• Dynamic, hollow tubes made of the
spherical protein tubulin
• Determine the overall shape of the cell
and distribution of organelles
Microfilaments
• Dynamic strands of the protein actin
• Attached to the cytoplasmic side of the plasma
membrane
• Braces and strengthens the cell surface
• Attach to CAMs (cell adhesion molecule) and
function in endocytosis and exocytosis, as well as the
“crawling” motion associated with amoeba
• Involved in the formation of cleavage furrows during
cell division
Microfilaments
Intermediate Filaments
• Tough, insoluble protein fibers woven like
rope and exhibit high tensile strength
• Stable, permanent cytoskeletal elements
• Serve as tracks for motility
• Attach to desmosomes and act as guy
wires to resist pulling forces exerted on the
cell
Intermediate Filaments
Centrioles
• Small barrel-shaped
organelles located in the
centrosome near the nucleus
• Pinwheel array of nine triplets
of microtubules
• Organize mitotic spindle during
mitosis
• Form the bases of cilia and
flagella
Cilia
• Whip-like, motile cellular extensions on exposed surfaces of certain
cells
• Move substances in one direction across cell surfaces
• E.g. in the respiratory tract, cilia move dust laden mucous away from
the lungs
Cilia
Figure 3.27a
Flagella
• Substantially longer cilia
• Only example in human is sperm
• Self propulsion
• Microtubules = 9 triplets
• Cilia/flagella = 9 doublets surrounding a
central pair
Nucleus
• Contains nuclear envelope, nucleoli,
chromatin, and distinct compartments rich
in specific protein sets
• Gene-containing control center of the cell
• Contains the genetic library with blueprints
for nearly all cellular proteins
• Dictates the kinds and amounts of proteins
to be synthesized
Nucleus
Figure 3.28a
Nucleus
• Most cells have a single nucleus, but
some are multinucleated and are often
associated with large cytoplasmic mass
and high protein production
• Mature red blood cells (RBCs) are
anuclated and do not divide (why?)
• Largest of all cytoplasmic organelles
• Divided into three regions:
–Nuclear envelope
–Nucleoli
–Chromatin
Nuclear Envelope
• Selectively permeable double membrane barrier
containing pores
• Encloses jellylike nucleoplasm, which contains
essential solutes
• Outer membrane is continuous with the rough
ER and is studded with ribosomes on its external
face
• Inner membrane is lined with the nuclear lamina,
which maintains the shape of the nucleus
• Pore complex regulates transport of large
molecules into and out of the nucleus
Nucleolus
• Dark-staining spherical bodies within the
nucleus
• Non-membrane bound structure
• Site of ribosome subunit production
Chromatin
• Threadlike strands of DNA and
histones
• Arranged in fundamental units
called nucleosomes (8 histones
wrapped twice w/ DNA)
• Histones help pack the long
strands of DNA and are involved
in gene regulation
• Form condensed, barlike bodies
of chromosomes when the
nucleus starts to divide
Figure 3.29
End of The Cell
• Let’s see that Harvard movie again and
see what we’ve learned
http://multimedia.mcb.harvard.edu/media.html
Cell Cycle
• Two major periods:
• Interphase
– Growth (G1), synthesis (S),
growth (G2)
• Mitotic phase (Cell
Division
– Mitosis and cytokinesis
Figure 3.30
Interphase
• Period from cell formation to cell division
• “Resting phase”: period between divisions
• During interphase, cell carries out all its routine
activities
• G1 (gap 1) – metabolic activity and vigorous
growth
• G0 – cells that permanently cease dividing
• S (synthetic) – DNA replication
• G2 (gap 2) – preparation for division
DNA Replication
• Replication begins simultaneously on
several chromatin threads & continues
until all DNA has been replicated
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
Steps in DNA Replication
• 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
Steps in DNA Replication (cont.)
• 7) After replication:
• histones associate w/ the DNA
• chromatin strands condense forming
chromatids and are held together by the
centromere
• at anaphase, chromatids are distributed to
each daughter cell
Steps in DNA Replication (cont.)
• 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
• This process is called semiconservative
replication
• Two new telomeres are also synthesized
DNA Replication
Figure 3.31
http://video.yahoo.com/watch/648006/30174
82
• Video of DNA Replication
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
From DNA to Protein
Nuclear
envelope
Transcription
DNA
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Polypeptide
Figure 3.33
Roles of the Three Types of
RNA
• 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
Genetic Code
• Rules by which base sequences of a gene
are translated into an amino acid
sequence.
• 2 Major steps:
Transcription
Translation
Transcription
• 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
Transcription: RNA Polymerase
• An enzyme that oversees the synthesis of
RNA
• Unwinds the DNA template
• Adds complementary ribonucleoside
triphosphates on the DNA template
• (5’ to 3’ direction)
• Joins these RNA nucleotides together
• Encodes a termination signal to stop
transcription
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
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)
Figure 3.34
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
Genetic Code
• RNA codons
code for amino
acids according
to a genetic code
Figure 3.35
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 nucleus
Transcription
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
Translation
• Anticodons form hydrogen bonds w/
complementary codons (base pairing)
• tRNA is the link (translator) between
nucleic acids and amino acids
• There are 45 different tRNAs each
capable of binding to a specific amino acid
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
Nucleus
Nuclear membrane
RNA polymerase
Nuclear pore
mRNA
Template strand
of DNA
Amino acids
Released mRNA
1
After mRNA processing, mRNA
leaves nucleus and attaches to
ribosome, and translation begins.
tRNA
Aminoacyl-tRNA
synthetase
Small ribosomal
subunit
Codon 15 Codon 16 Codon 17
Direction of
ribosome advance
Portion of mRNA
already translated
tRNA “head”
bearing
anticodon
Large
ribosomal
subunit
2
4
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.
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
Information Transfer from DNA
to RNA
Figure 3.38