Transcript DNA and Genes - Perkins Science

Cell Nucleus and Gene
Expression
LECTURE 7 – CHAPTER 3
• Most cells have one
nucleus.
a.Muscle cells have
hundreds;
b. mature RBCs have none.
CELL NUCLEUS
Two membranes
Outer membrane fused with rough ER
Inner membrane continuous with outer membrane
DNA and Genes
The nucleus contains DNA. A gene is a length of DNA that
codes for a specific protein.
Transcription –
DNA is used as a code
to make mRNA
Translation –
mRNA is used as a
code to make protein
These two steps can
be called Gene
expression.
Nucleoli
The nucleus also has one or
more darker regions not
surrounded by a membrane;
these are called
nucleoli.
Singular =
Nucleolus
The nucleoli contain the DNA that codes for
the production of ribosomal RNA.
Genome and Proteome
Genome - all the genes
in a particular individual
or species
-humans have ~23,000
genes.
Proteome - all the
proteins that are
produced from the
genome.
Chromatin = histones bound to DNA
Nucleosome – 2 turns of DNA around histones
Euchromatin - active in transcription, looser.
Heterochromatin - inactive regions, highly condensed.
ACETYLATION OPENS UP CHROMATIN FOR TRANSCRIPTION
What is a Gene?
A GENE CONTAINS:
1) Start and stop regions
2) Promoters, areas of DNA that are not part of
the gene but tell enzymes involved where to
begin
Transcription factors bind to the promoter to begin
transcription
RNA Synthesis
RNA polymerase
assembles the
growing mRNA
--RNA nucleotides
pair up to the DNA
template
--Assembly is
complementary.
--only 1 strand is
transcribed
4 Types of RNA
a.Precursor messenger RNA (pre-mRNA)
b.Messenger RNA (mRNA) –
c.Transfer RNA (tRNA) –
d.Ribosomal RNA (rRNA) –
Transcription and RNA Modification in the Nucleus
Exons are joined together by spliceosomes and snRNPs
RNA Interference
RNA molecules may
prevent some mRNA
molecules from being
translated.
-- siRNA
(short interfering)
and miRNA (micro
interfering)
-- The expression of at
least 30% of genes
is regulated in this way.
Protein Synthesis (Translation)
• mRNA attaches to a string of ribosomes
to form a polyribosome.
• The order of codons give the sequence of
amino acids
TRANSCRIPTION & TRANSLATION:
Transfer RNA (tRNA)
• Cloverleaf shape
• Further twisted into an
upside down L
• Aminoacyl-tRNA
synthetase adds an amino
acid to the “accepting end”
• If the anticodon is
complementary to the
codon, the amino acid will
be inserted.
FORMATION OF A POLYPEPTIDE CHAIN
Chaperones help the
new protein fold
How Secretory Proteins Enter the ER
A leader sequence is formed, which allows the protein to
go through the hydrophobic membrane into cisternae
Formation of insulin hormone
Figure 3.23
1) preproinsulin enters
the cisterna (109aa)
2) Leader is removed,
producing proinsulin
(86aa)
3) Central region is
removed, leaving a 21
and 30 aa hormone
that is two chains.
NOTICE THE
DISULFIDE
BRIDGES
Secretory proteins are next sent to the Golgi complex.
a. Proteins may be further
modified.
b. Proteins are
- separated according
to destination.
- packaged and
shipped in vesicles to
their destinations.
The Ubiquitin-Proteasome System
-
DNA Synthesis
DNA Replication
DNA REPLICATION
HELICASE
DNA POLYMERASE
5’  3’ SYNTHESIS
SEMICONSERVATIVE
REPLICATION
THE CELL CYCLE
Interphase: G1
If a cell does not divide, it remains in a modified G1
phase its whole life.
The cell is performing the
functions characteristic of cells in that
tissue.
- Cyclin D moves the cell through G1.
- p53- a tumor suppressor gene –
inhibits cancer
How do they know? Read p. 75, about knockout
mice. Turn to your neighbor and explain how it’s
done
S-Phase and G2
S phase: “SYNTHESIS”
G2 phase: Chromosomes
start to condense; consist
of two strands called sister
chromatids joined by a
centromere.
A CHROMOSOME
AFTER DNA
REPLICATION
“Condensed”
Cell Death
Necrosis: Cell DEATH due to deprivation of blood
supply.
Apoptosis: Programmed cell death is performed
by enzymes called caspases.
1)Extrinsic Apoptosis:
“Death ligands”
and their receptors
2)Intrinsic Apoptosis:
Intercellular
signals trigger death
Interphase –
Prophase –
nuclear membrane
disappears,
spindle forms
Metaphase –
Chromosomes line
up in center
and attach to
spindle fibers
Anaphase –
Centromeres
split, spindle
fibers shorten
& pull
chromatids to
opposite sides
3.TelophaseCytoplasm is
divided
(cytokinesis),
nuclear
membranes reform, cells split.
Role of the Centrosome
-forms the spindle
-helps form cleavage furrow
during cytokinesis
Non-dividing cells: centrosome
migrates to the plasma
membrane and forms the
nonmotile primary cilium.
-In ciliated cells, hundreds of
centrosomes form and become
the basal bodies of the cilia
Looks like a churro!
Telomeres and Cell Division
a.Telomere loss may signal
end of cell division
b.Damaged telomeres
activate p53 which induces
cell cycle arrest,
senescence, and apoptosis
c. Cells that can divide
indefinitely, (e.g. bone
marrow), have telomerase
that replicates the
telomere.
Telomeres –
ends of chromosomes
Hypertrophy and Hyperplasia
Hyperplasia: growth
due to an increase in
the number of cells;
Hypertrophy:
growth due to an
increase
in cell size;
Karyotype –
Chromosomes
are arranged
in homologous
pairs
Humans have
23 pairs
1 pair – sex
22 - autosomes
Meiosis I –
“reduction division”
Meiosis – how cells
make gametes
Prophase I:
Homologous chromosomes
pair up.
Crossing over is
a source of
genetic variability
parts are swapped
in crossing-over.
Metaphase I - Homologous
chromosome pairs line up at the
equator [chromosomes shuffle]
Anaphase I - Homologous
chromosomes are pulled apart.
Telophase I - Homologous
chromosomes are separated.
This results in two daughter
cells with 23 chromosomes
each
Prophase II –
spindles form
Metaphase II –
sister chromatids line up
in center
Anaphase IICentromeres are broken,
chromosomes pull apart
Telophase II –
4 cells with 23
chromosomes each
For animations, click here
Meiosis Summary
Epigenetic Inheritance
• When silenced genes are passed to daughter cells
during mitosis and meiosis without a change in DNA
base sequence.
• Mechanisms of epigenetic inheritance
a.Post-translational modifications of histones
b.Methylation of Cs before Gs
c.Acetylation of histones