Download Cells: The Living Units Part 2

Chapter 3 – Part 2– Cell Cycle
o Interphase
o G1 (gap 1) – metabolic activity and vigorous growth
o G0 – cells that cease dividing
o S (synthetic) – DNA replication
o G2 (gap 2) – preparation for division
o Mitotic phase
o Mitosis and cytokinesis
Interphase – G0
o G0 represents not simply the
absence of signals for mitosis
but an active repression of the
genes needed for mitosis.
o Cancer cells cannot enter G0 and are
destined to repeat the cell cycle indefinitely
o Most cells will leave the cell cycle, temporarily. They remain in G1 until
properly stimulated to begin dividing again.
o Often G0 cells are terminally differentiated: they will never reenter the cell
cycle but remain in G1 until they die. (muscle and nerve cells)
Interphase – S phase
(DNA replication)
o _________untwists
the double helix
Helicase
and exposes complementary strands
o Each nucleotide strand serves as a template
for building a new ______________
complementary strand
RNA primers
o _____________are
added to begin DNA
synthesis
DNA polymerase III
o__________________continues
from the primer
and covalently adds complementary __________
nucleotides
to the template
o Since DNA polymerase only works in one
direction: 5` to 3`
o A continuous leading strand is synthesized
o A discontinuous lagging strand is
synthesized
o __________
DNA ligase splices together the short
segments of the discontinuous strand
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Mitosis – Nuclear Division
o Essential for body growth and tissue repair
o The phases of mitosis are:
o Prophase
o Metaphase
o Anaphase
o Telophase
o Cytokinesis – division of the cytoplasm
o Cleavage furrow formed in late anaphase by
contractile ring
o Cytoplasm is pinched into two parts after
mitosis ends
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Early and Late Prophase
o Asters are seen as chromatin
condenses into chromosomes
o Nucleoli disappear
o Centriole pairs separate and the
mitotic spindle is formed
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Metaphase
o Chromosomes cluster at the middle of the cell
with their centromeres aligned at the exact
center, or equator, of the cell
o This arrangement of chromosomes along
a plane midway between the poles is called
the metaphase plate
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Anaphase
o Centromeres of the chromosomes split
o Motor proteins in kinetochores pull
chromosomes toward poles
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Telophase and Cytokinesis
o New sets of chromosomes extend
into chromatin
o New nuclear membrane is formed
from the rough ER
o Nucleoli reappear
o Generally cytokinesis completes cell division
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Control of Cell Division
o Surface-to-volume ratio of cells
o Contact inhibition
o Chemical signals such as growth
factors and hormones
o Cyclins and cyclin-dependent kinases
(Cdks) complexes
Cell destruction – death
o Necrosis "cell homicide" -Inflammation with secondary injury to surrounding
normal tissues..
o Apoptosis "cell suicide". -No Inflammation or secondary tissue injury
Control of Cell
Reproduction
o Most cells divide an average of about
50 times
o Mitotic clock -tips of chromosomes
(telomeres) about 12,000 nucleotides
o
25-200 nucleotides lost with each
division
o When repeats are gone cell
quits dividing.
o Cell Differentiation - when cells develop
different structures and specialized functions.
o Stem cell can divide to form a progenitor cell
o totipotent – can give rise to any cell type
o pluripotent – can give almost any cell type in the body
o multipotent – can differentiate into a limited range of cells
Loss of Cell Division Control - Cancer
o Division to frequent dysplasia neoplasmtumor
o benign -->lump in place
o malignant - lump extends into
surrounding tissue
o resembling a crab -->metastasis
o Genes that cause cancer
o oncogenes activate other genes that
increase cell division
o tumor suppressor gene – normally
regulate mitosis; if inactivated they will not
regulate mitosis
o Environmental causes?
o
Cancer treatments are directed at
procedures that kill fast dividing cells.
What normal cells do they also effect?
Protein Synthesis
o DNA serves as master
blueprint for protein synthesis
o Genes are segments of DNA
carrying instructions for a
polypeptide chain
o Triplets of nucleotide bases form
the genetic library
o Each triplet specifies coding for
an amino acid
Transcription
o Transfer of information from the sense strand of DNA to RNA
o Transcription factor
o Loosens histones from DNA in the area to be transcribed
o Binds to promoter, a DNA sequence specifying the start site of RNA
synthesis
o Mediates the binding of RNA polymerase to promoter
o DNA triplets are transcribed into mRNA codons by RNA polymerase
Translation to Polypetide
Roles of the Three
Types of RNA
o Form Initiation complex
o Messenger RNA (mRNA)
carries the genetic information
from DNA in the nucleus to the
ribosomes in the cytoplasm
o Transfer RNAs (tRNAs)
bound to amino acids base
pair with the codons of mRNA
at the ribosome to begin the
process of protein synthesis
o Ribosomal RNA (rRNA) is a
structural component of ribosomes
Information
Transfer from DNA
to RNA
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Centrioles
o Small barrel-shaped organelles located in
the centrosome near the nucleus
o Pinwheel array of nine triplets of microtubules
o Form the bases of cilia and flagella
o Organize mitotic spindle during mitosis
kinetochore
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