D2.1 Cell and Nuclear Division Flashcards
Reasons why cells need to divide
- Repair
- Replace
- Growth
Define Cytokinesis
Division of the cytoplasm after nucleus divided through meiosis or mitosis. Last part of division of cell: physically splitting the two apart.
Explain Cytokinesis in animal cells.
- Network of actin and myosin proteins form a contractile ring that pinches the cell membrane together.
- This forms a cleavage furrow, which gradually deepens and eventually splits the cytoplasm to form daughter cells.
- Fluid structure of double membrane facilitates this process.
Explain Cytokinesis in plant cells.
- Involves assembly of a cell plate, formed from the fusion of vesicles containing cell wall materials.
- Cell plate grows outwards until it reaches existing cell wall, which it fuses with, splitting the parent cell into two daughter cells.
Explain unequal cytokinesis
Usually during cytokinesis, cytoplasm is split equally between the new cell but sometimes one cell can have a larger cytoplasm. This can only occur if both cells receive a nucleus and at least one mitochondria.
Explain Budding in yeast as an example of unequal cytokinesis
Budding in yeast:
a type of asexual reproduction.
Parent cell replicates nucleus and the budding receives one and just enough cytoplasm to have maybe one copy of each organelle and then a new cell wall will grow between them to separate.
Explain Oogenesis as an example of unequal cytokinesis
Oogenesis is the production of eggs (eggs can be called oocytes).
Made by meiosis.
Meiosis has two cell divisions: first is unequal: one smaller and one bigger.
Smaller one divides into 2 polar bodies.
Bigger one divides into another polar body and one viable oocyte.
Outline the cause and consequence of anucleate cells.
Cause: to fit function - E.g. red blood cell needs maximum space for hemoglobin so nucleus is lost during maturation.
Consequences:
- Anucleate cells cannot synthesize proteins or replicate DNA, as they lack the necessary genetic material. This limits their ability to respond to environmental changes or repair themselves.
- short lifespan
Explain Mitosis
Nuclear division
Represents continuity: produces two genetically identical daughter cells.
Each of the daughter cells are diploid 2n that means chromosomes come in pairs.
All cells have complete copy of the genome.
Explain Meiosis
Nuclear division
Represents change: produces four genetically unique daughter cells.
All daughter cells are haploid n.
Meiosis halves the chromosome number.
There is a random assortment of genes which results in variation
Explain when DNA replication occurs
Before mitosis and meiosis, in S-phase of interphase.
DNA is replicated and condenses into chromosomes.
Explain how replicated DNA molecules are held together, with reference to chromatid, replicated chromosome, centromere and cohesin.
Before replication: chromosome consisting of one chromatid and a centromere.
After replication: chromosome with two chromatids that are held together by cohesin loops.
(these chromosomes are pulled apart during anaphase.)
Explain how and why chromosomes condense during mitosis and meiosis.
Condensation of chromosomes helps move DNA safely and efficiently. To do this, Supercoiling: DNA is wrapped around histone proteins which then link together.
Role of Microtubules and kinetochore motor proteins
Once DNA is condensed, they need to move towards the ends of the poles of the cells so that it can eventually become the two new daughter nuclei.
Cytoskeleton is made up of protein filaments called microtubules and they are going to be disassembled and reassembled to become spindle microtubules. Looks like puppet strings.
They link with a structure called the kinetochore on the centromere.
Kinetochore will act as a microtubule motor, moving the chromatids to opposite poles by removing the microtubule filaments one at a time, making the microtubules shorten, which pulls the chromatids apart to opposite ends of the cell.
Phases in Interphase
G1 phase: cell grows.
S phase: DNA synthesis: chromosomes are copied.
G2 phase: Preparation for division: cell grows and produces organelles, proteins and membranes.
Phases of mitosis
Prophase
Metaphase
Anaphase
Telophase
Why is Meiosis considered a reduction division?
Meiosis reduces the chromosome number by half so that means that the gametes will have 23 chromosomes and not 46. Since gametes have 23 chromosomes, during reproduction, the zygote made by the fusion of the gametes will result in a 46 chromosome zygote.
Stage prior to mitosis
Interphase:
DNA in chromatin form.
Simply explain the process of Meiosis
Production of gametes:
DNA replicated once.
Cell divided twice: Meiosis I and Meiosis II.
Forms 4 haploid cells with 23 chromosomes each.
Describe the stage of Prophase.
Chromatin condensed into chromosomes.
Spindle microtubules start to assemble.
Nuclear membrane breaks down.
Define Nondisjunction and Explain how Down syndrome occurs
Failure of chromosomes to separate. Results in gametes with too many/few chromosomes.
Usually results in cell death.
However, in 23rd chromosome, cell does not self-destruct and instead results in down syndrome. Three chromosomes in the 21st pair.
Highly correlated with maternal age: almost exponential increase in chance of this occurring after age 35
Describe the stage of Metaphase
Microtubules attach to the centromeres.
Chromosomes align on the middle.
Nucleus has fully dissolved
Factors that affect the diversity in sexually-reproducing organisms
Mutation
Meiosis
Fertilization
Describe the stage of Anaphase
Cohesin loops that connect the sister chromosomes are cut
Sister chromosomes are pulled apart and now each pole has a chromosome.
