Lecture 11 & 12 Cell cycle and cell division Flashcards
Why is cell division important?
-Continuity of life
• Passes genetic information to the next generation
• Universal – unicellular to multicellular
• Long, complex series of cell divisions required
• Replace cells that die
How is cell division acheived?
• AIM: To produce two genetically identical daughter cells • Replication of DNA in each chromosome • Duplicate organelles • Two complete copies accurately distributed between daughter
What is mitosis?
Mitosis:
- Asexual reproduction
- Diploid (two complete chromosome sets)
- Homologous copies
- Daughter cells are genetically identical
After which phase in the cell cycle does mitosis come?
- After interphase (resting phase)
- interphase made up of G1, S, G2
What happens in the G1 phase of the cell cycle?
- AIM: Recover from M Prepare for S phase
- Gap/Growth Phase 1
- General DNA checking/repair
- Synthesis of RNA and protein
- Generation of new organelles
- Increase in cell size
What happens in S phase of cell cycle?
• AIM: To replicate genome
• Synthesis phase
• Chromosome duplication
(DNA= histones and chromatin = chromosome)
What are cohesins?
• A large protein complex
• Deposited at many positions along the sister
chromatids during DNA replication
• Holds sister chromatids together
What is karyotyping?
-occurs doing metaphase
• Species variation: Humans – 23 pairs of homologous chromosomes (2n)
• shows Size, shape, position of centromere
• Alternations could lead to genetic disorders
What happens in the G2 phase of the cell cycle?
- AIM: To prepare for mitosis
- Gap/Growth Phase 2
- Second round of DNA proofreading
- RNA and protein synthesis resumes
- Cell confirms the number of chromosomes and the organelles present
- Microtubule synthesis
What happens during the M phase of the cell cycle?
• AIM: Cell division • Mitosis • Divide sister chromatids • Divide the nucleus • Division of the cytoplasm (Cytokinesis) Mitosis: • Asexual reproduction • Diploid (two complete chromosome sets) • Homologous copies • Daughter cells are genetically identical
What are the phases of mitosis?
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
What happens in Prophase?
• Chromosomes replicated in S phase condense (two sister chromatids)
• Mitotic spindle assemble between the two centrosomes:
+ Centrosomes – organelle, microtubule organising centre
+ Centrosome = centrioles + microtubules
What happens in Prometaphase?
- Nuclear Envelope breaks down
- Chromosomes attach to spindle microtubules via kinetochores (Cohesin laid down during S phase are removed)
- Chromosomes undergo active movement
What happens during metaphase?
• Chromosomes are aligned at the equator of the spindle, midway between the spindle poles
• The kinetochore microtubules attach sister chromatids to opposite poles
of the spindle
What happens during anaphase?
- Sister chromatids synchronously separate to form two daughter chromosomes
- Each is pulled slowly towards the spindle
- Kinetochore microtubules get shorter, spindle poles move apart to complete chromosome segregation
What happens during telophase?
- Two sets of daughter chromosomes arrive at the spindle poles and decondense
- New nuclear envelope reassembles around each set of chromosomes
- Two nuclei are formed marking the end of mitosis
What happens during cytokinesis?
- Cytokinesis starts in late anaphase (plants are different to animals)
- Contractile ring of actin and myosin filaments pinches the cell in two
- Two daughter cells are created each with one nucleus and three decondensed chromosomes
How is cytokinesis different in plants?
- Plant cells cannot form a contractile ring
- Assemble a plasma membrane and cell wall between two daughter nuclei
- Cell Plate
How has our understanding of the cell cycle progressed?
- Model organisms
- Unicellular
- Multicellular
What is binary fission?
- Unicellular organisms
- Mitosis (asexual reproduction)
- Cell elongates and DNA is replicated
- Cross wall forms
- Identical clone daughter cells
- Adapt to new conditions?(only sometimes)
How is yeast used to study the cell cycle?
Regular yeasts:
• Fission yeast S. pombe has a typical eukaryotic cell cycle
• Rod shape grows by end elongation
• Exhibits G1, S, G2 and M phases
• The budding yeast S. cerevisiae has a different cell cycle
• G1, S and M phases, but does not have G2 phase
Mutant yeasts:
• Rapid reproduction, small genome
• Haploid (single gene copy)
• Easy genetic manipulation: deletion, replacement, alteration
• Discovery of cell-division-cycle genes (cdc genes)
-CDC sensitive mutants:
• Search for cdc genes – ‘cell cycle arrest’ at a specific phase
• Conditional temperature sensitive mutation
• Continue through cell cycle until inactivation gene required
• Uniform cell cycle arrest
How are embryonic frog cells used to help further our understanding of multicellular cell cycle?
- Important model for studying cell cycle
- Large cells (1mm diameter)
- Easy to inject test samples
- Large cytosol volume (contains proteins involved in cell cycle)
- Rapid sequence of cell divisions - CLEAVAGE DIVISIONS
- Divided into S and M phases (15 mins each)
- No detectable G1 or G2 phases
How is the role for chemical signals in the cell cycle tested for?
- Cell fusion experiments
- Fuse S phase and G1 phase
- DNA synthesis initiates in G1 nucleus
- Substance in S phase cell responsible? (perhaps)
- Fuse M phase nucleus with G1 (interphase) cell
- Cell G1 cell immediately enters mitosis
- Chromosome not replicated (no S phase)
- But, spindle formed, chromatin condensed and nuclear envelope breakdown occurs
- Condensed chromosome unduplicated
What are the key transition points in the cell cycle?
- G2=M transition
- Metaphase=Anaphase transition
- G1= restriction point (if it doesnt go on to duplicate)
What are the controllers of transition points?
• Cyclin dependant kinase (CDKs) – inactive (anaphase promoting complex APC)
• Cyclins:
+ Four types – G1, S, G2 and M
+ Synthesised at specific cell cycle phases
+ Protected from degradation and accumulate
+ Cyclically degraded
• Cyclins/CDK complex (regulate transitions)
• MPF (maturation promoting factor)
• GO signal for transition points
How can CDK inhibitors (CDKIs) suppress CDK activity?
- Tumour suppressor genes
- Block CDK phosphorylation activity
- Arrest cells – frequently at G1 phase (G0)
For what reason would a transition be blocked?
-restriction point (G1) \+Growth factors, nutrients \+Cell Size \+DNA Damage -G2=M transition \+Cell size \+DNA Damage \+DNA replication -Metaphase-Anaphase transition \+Chromosome attachments to spindle
What are some extrinsic factors that can affect the cell cycle?
- Mitogens
- Platelet-derived growth factor
- Anchorage-dependent growth
- Density-dependent growth
- Cell cycle blocked even in presence of growth factors
What is meiosis?
Sexual reproduction:
• HAPLOID cells (n) – carry only a single copy of the chromosome
• Gametes (eggs, sperm) are haploid cells
• When an egg and sperm fuse a DIPLOID zygote is formed
• DIPLOID CELL (2n) – two slightly different copies of the same chromosome one from each parent
• Genomes of two parents mix to generate
genetically distinct offspring
Describe the process of Meiosis?
• Starts with a DIPLOID zygote – two slightly different copies of the same chromosome one from each parent
• Meiosis generates HAPLOID daughter cells – carry only a single copy of the chromosome
• Reduces chromosome number
• Often differentiate into gametes (eggs, sperm)
-Meiosis 1
-Meiosis 2 (see other lecture for details)
Meiosis 1 contributes to genetic variation:
• Crossing over produces recombinant chromosomes
• Humans – 23 chromosomes = 223 = 8.4 million combinations
• Egg + sperm = zygote = 223 x 223 = 70 trillion combinations
What is non-disjunction?
• Abnormal sister chromatid separation, causes:
• Aneuploidy – different chromosome number from normal
• Complete or partial omission of X chromosome
-Trisomy
