Cell Biology Chapter 7 - The Nucleus and DNA Replication Flashcards
Key concepts – DNA Integrity is KEY!
The Nucleus Contains and Protects Most of the Eukaryotic Cell’s DNA
Key concepts – DNA Integrity is KEY!
The nucleus is a highly specialized organelle committeed primarily to protecting, copying and transcribing DNA
The interior of the nucleus is highly compartmentalized
DNA copying, plus transcribing and splicing of RNA, are accomplished by large, highly specialized molecular complexes
The Nuclear Envelope Is A Double Membrane Structure
the nuclear envelope encloses the nucleoplasm outer membrane of the nuclear envelope is continuous with the ER
Evolution of the nucleus
The nucleus may have arisen by endosymbiosis, a process in which one prokaryotic cell engulfs another cell, which then becomes a primitive nucleus.
The Outer Membrane of the Nuclear Envelope is Continuous with the Endoplasmic Reticulum
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Nuclear Pore Complexes Regulate Molecular Traffic Into And Out Of The Nucleus
Key Features
layers of rings stacked on top of one another that span both nuclear membranes, linked to filamentous protein fibrils to form a basket structure
Pores are fomed at the places where the inner and outer membranes fuse together (not yet known how this happens)
Structure undergoes complex conformational changes when it transports material into and out of the nucleus
The Interior Of The Nucleus Is Highly Organized And Contains Subcompartments
Nucleolus contains DNA that encodes ribosomal rRNAs
Nucleoli are sites of high transcriptional activity for rRNA genes
Sometimes there are more than one per cell
Nucleoli do not have a membrane – they are part of the nucleus
The Nuclear Matrix Helps To Organize Chromosomes
Nuclear organization is not random
Chromosomes are compartmentalized into regions called chromosome territories
Nuclear matrix helps control the shape of chromosomes and regulate heterochromatin and euchromatin
Not known to contain any other cytoskeletal elements other than Nuclear Lamins.
: Individual chromosomes occupy distinct areas of the nucleus called chromosome territories
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The nuclear matrix is a network of filaments bound to the nuclear envelope and to DNA.
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DNA Replication Occurs At Sites Called Replication Factories
DNA replication factories form large complexes in the nucleus devoted to copying DNA with 100% accuracy and no breaks
Replisome is the smallest functional unit in the factories and are responsible for copying one segment of DNA
RNA Polymerase Complexes And Spliceosomes Are Distinct Structures Within The Nucleus
RNA polymerase complexes are responsible for transcribing the DNA sequence in genes into mRNA, rRNA, tRNA, and other RNAs
Spliceosomes are responsible for splicing the newly synthesized RNAs into their mature form
DNA Polymerases
Are Enzymes That Replicate DNA
DNA polymerases
DNA polymerases add deoxyribonucleotides to the 3’ end of DNA strand
DNA polymerases proofread their work
Have an exonuclease domain
DNA Replication Is Semi-discontinuous
DNA replication begins at sites on chromosomes called origins of replication
E.coli have only one
Eukaryotes have several on each chromosome
What happens at the origin of replication in eukaryotes
hexameric helicase
During Replication, Specialized Proteins Unwind And Separate The Two Strands To Form A Replication Fork
DNA Replication Is Semi-discontinuous
If DNA builds DNA from a single-stranded template, how can it require a double strand to get started?
DNA replication requires an RNA primer (synthesized by primase)
leading/lagging strand
Okazaki fragments
DNA ligases join fragments of single-stranded DNA
The helicase creating the replication fork is connected to
two DNA polymerase catalytic subunits, each of which is held onto DNA by a sliding clamp
DNA Ligases
Join Fragments of Single-Stranded DNA
Replication Of DNA At The End Of Chromosomes Requires Additional Steps
Role switching
Reverse Transcriptase Activity
Cells Have Two Main DNA Repair Mechanisms
Excision repair systems
Mismatch repair
Recombination repair
Recombination Systems Repair Large-Scale DNA Damage
Necessary as a last-ditch effort to repair DNA damage prior to cell division
Only rely on recombination to fix damage
Occurs either in the metaphase stage of mitosis or in meiosis
Arms of chromosomes occasionally break and swap pieces
Mitosis Is Divided Into Stages
1879 - Walther Flemming described the motion of what he saw under microscope as “threads” (Greek, mitos) moving in an actively dividing cell
Prophase
Prepares The Cell For Division
Gene Expression Halts
Motors Contribute To The Formation Of The Mature Spindle In Prophase
Dynein motor proteins
Kinesin-related motor proteins (Eg5)
Chromosomes Attach To The Mitotic Spindle During Prometaphase
Kinetochores attach chromosomes to the mitotic spindle
Arrival Of The Chromosomes At The Spindle Equator Signals The Beginning Of Metaphase
Dynamic microtubules search for kinetochores throughout the cell by growing and shrinking in random directions from the centrosomes
Metaphase plate = spindle equator
Chromosome recombination takes place during metaphase
Separation Of Chromatids At The Metaphase Plate Occurs During Anaphase
The onset of anaphase requires dissolving the connections between sister chromatids Anaphase Promoting Complex Anaphase is subdivided into two phases: anaphase A anaphase B
Figure 07.24:
Separation of replicated chromosomes during anaphase.
Telophase - Cytokinesis
The structural rearrangements that occur in prophase begin to reverse during telephase
Cytokinesis completes mitosis by partitioning the cytoplasm to form two new daughter cells
Formation of a cortical actin-myosin filament network called the contractile ring
Fragmentation of non-nuclear organelles ensures their equal distribution in the daughter cells
For a brief moment, the plasma membrane between the two halves is ruptured, but it reseals almost instantaneously
