Lecture 12 Flashcards
What happens when a cell reaches a certain size?
It either divides or stops growing
What is the range of time it takes for a cell to go through a cell cycle?
8-20 hours
What are the two main phases in the cell cycle?
Interphase and M phase
Interphase?
When the cell is growing and actively metabolising
- G1
- S phase (chromosomes duplicate)
- G2
M phase?
When the cell is actively dividing
- Prophase
- Metaphase
- Anaphase
- Telophase
Two types of chromatin?
Heterochromatin and euchromatin
Heterochromatin?
- Condensed
- DNA is inactive
Euchromatin?
- Unravelled
- Genes are expressible (DNA is active)
What happens to the chromatin in the early stages of mitosis?
They become more and more condensed
Centromere?
- A fix point in the centre of the chromosome that holds the two chromatids together
- Holds the chromosome in its shape
- Controls the movement of the chromosomes
Are the sister chromatids identical?
Yes, bc they each have the same DNA
Kinetochore?
- During late pro-metaphase when the 2 sister chromatids are separated from one another, an extra plate formed of protein is found at the centromere. This is the kinetochore, and is where the microtubules attach when pulling the chromatids apart
- Control assemble and disassembly of spindle fibre microtubules
- One on each chromatid
Where is the telomere located?
At the ends of the chromatids
What is the function of the telomere?
- Protect end of chromosome from deterioration or fusion with neighbouring chromosomes
- Maintain the structural integrity of the DNA
- Ensures complete DNA replication with the enzyme telomerase
- Position chromosomes in the nucleus
Structure of the telomere?
- DNA and protein that cap the ends of the chromosome
- 5 - TTAGGG - 3 repeating DNA sequence (repeated over 1000 times)
Karyotype?
A chart that orders the homologous chromosome pairs by number, size and morphology (biggest to smallest)
Gap 1?
- High levels of RNA transcription and protein synthesis
- Prepares to duplicate DNA
- Continuation of cytokinesis
- Increasing number of organelles such as mitochondria
S phase?
- DNA synthesis and replication
- RNA transcription and protein synthesis is continued
Gap 2?
- DNA has replicated in preparation for division
- Further RNA transcription and protein synthesis occurs
Quiescence or G0?
- After a long period of interphase when cells temporarily or reversibly stop dividing
- State of non-division
- Cells enter the G0 phase at an early stage in the G1 phasw
Senescence?
When cells permanently stop dividing due to age or accumulated DNA damage (apoptosis = cell death)
What are the 3 cell cycle check points?
- G1 (restriction point) just before entry into the S-phase
- G2 just before entry into mitosis
- Metaphase - if the chromosomes don’t line up correctly then it doesn’t proceed with mitotic anaphase
What protein regulates G1 as well as the formation of cancer cells?
p53
What ultimately happens in mitotic phase?
The division of a mother cell into two daughter cells
What is the purpose of the 3 cell cycle check points?
- To make sure that only healthy cells go on to produce daughter cells
- Double checks to make sure everything is okay
Mitosis?
- A mother cell divides to produce two identical daughter cells
How are chromosomes produced?
- Chromatin fibres in the nucleus condense as the cell is about to divide
- This forms sister chromatids
- The sister chromatids come together to form a chromosome and are attached at the centromere
What does the process of chromatin fibres condensing involve?
Extensive folding and looping
Karyokinesis?
What happens to the chromosomes in the nucleus during cell division
Cytokinesis?
The division of the cytoplasm
What are the two overlapping processes of cells separating its duplicated genome into two identical halves?
Cytokinesis and karyokinesis
What is the first thing that starts ti breakdown going from early prophase to late prophase?
The nuclear envelope
Early prophase?
- mitotic chromosomes appear
- each chromosome is seen to be divided into 2 sister chromatids, which are held together at the centromere
- the centrioles, which duplicated during S phase, now move with their centrosomes to opposite poles of the cell
Late prophase?
- division of the chromosomes into chromatids is now apparent
- a cluster of microtubules (an aster) extends from the centrioles and begins to form the mitotic spindle
- the nucleoli disappears
Pro-metaphase?
- nuclear envelope breaks down
- microtubules invade the nuclear space
- kinetochores form on the chromosomes
- polar microtubules from each ester overlap, forming interconnections
- microtubules start to search for the kinetochores
Metaphase?
- chromosomes align in the centre of the spindle on the metaphase plate
Anaphase?
Step 1:
- the kinetochore proteins are cleaved, allowing the sister chromatids to separate
- these chromatids are now called sister chromosomes and are pulled apart through the shortening of the kinetochore microtubules
- they’re pulled towards their respective centrosomes on opposite poles
Step 2:
- the polar kinetochore microtubules elongate, pushing the centrosomes (and the set of attached chromosomes) apart to opposite ends of the cell
Telophase?
- begins when the chromosomes reach the opposite poles
- mitotic spindles break down
- polar microtubules further elongate
- new nuclear envelope forms around the 2 clusters of daughter chromosomes from the ER nearby
- new nucleoli is reconstructed
- chromosomes partially uncoil
- chromosomes then fully uncoil to form chromatin fibres that become invisible
Cytokinesis?
- cytoplasm constricts where the metaphase plate used to be (furrowing)
- the contractile material is a belt of actin microfilaments, which forms a noose-like ring
- as the actin microfilaments constrict, the cleavage furrow deepens and the cells separate
Cyclins?
Proteins that control cell cycle at each checkpoint
What is a predominant feature of cancer cells?
Uncontrolled cell growth
What is uncontrolled cell growth due to?
- Cyclin levels are higher than normal
- p53 tumour suppressing protein is usually absent at the G1 phase, allowing progression into the S-phase
What do cyclins activate?
Protein-dependent kinases (Cyclin-dependent kinases…Cdks)
What do cyclins activate?
Protein-dependent kinases (Cyclin-dependent kinases…Cdks)
Cyclin-dependent kinases?
Enzymes that can either activate or inactivate other proteins through phosphorylation
What is the combination of cyclins joining with Cdks?
A cyclin-Cdks complex
What does the cyclin-Cdks complex do?
- Activates other proteins, which are needed in the G1 point of the cell cycle
- Positive regulation of this complex allows for progression into the next phase of the cycle
