Cell Division: Replication and Meiosis

• Cell division leads to growth in multicellular organisms

• It supports reproduction, repair, and genetic diversity

• DNA replication must occur before the nucleus divides

• Daughter cells need the same number of chromosomes as the parent, achieved through replication

• The nuclear division that achieves this is called mitosis

• Sexual reproduction involves gametes fusing to form a zygote

• This requires a division that halves the number of chromosomes, called meiosis or reduction division

The Cell Cycle

• The cell cycle comprises the events between one cell division and the next

• The cycle has three main stages:

• Interphase: Where synthesis and growth occur, including DNA replication

• G1 phase: The cell grows, copies organelles, and makes molecular building blocks

• S phase: The cell synthesizes a complete DNA copy and duplicates the centrosome containing centrioles

• G2 phase: The cell grows more, makes proteins/organelles, and reorganizes for mitosis

• Mitosis (or Meiosis): Nuclear division occurs

• Cytokinesis: The cell splits into two daughter cells

Mitosis

• Mitosis is cell division that produces two genetically identical daughter cells from one parent cell

• Most cell divisions in the body involve mitosis

• In single-celled eukaryotes like yeast, it's a form of reproduction

• Mitosis ensures each daughter cell receives a full and identical set of chromosomes as the parent cell

• Cells with too few or too many chromosomes may not survive or can cause disorders like cancer

Significance of Mitosis

• Mitosis ensures genetic stability through the replication of chromosomes before cell division, creating clones
• Growth arises through the number of cells increasing through mitosis, the basis of growth in multicellular organisms
• Cell Replacement constantly sloughs off dying cells, replacing them with exact copies to retain normal function
• Regeneration allows some animals to regenerate body parts with new cells produced via mitosis
• Vegetative reproduction results in genetically similar offspring (clones) through asexual reproduction

Phases of Mitosis: Before Mitosis

• A cell is in interphase (late G2), during which DNA is copied into two sister chromatids

• Chromosomes appear de-condensed in this phase

• During interphase:

• Chromosomes are not clearly visible since they are de-condensed

• There are two copies of the centriole in animal cells, orchestrating mitosis

• Plants cells have a different microtubule organizing structure

Important Terms

• Centromere: A point of constriction on chromosomes that contain DNA sequences that bind specific proteins

• Kinetochore: A disc-like structure with proteins at the centromere site

• Cohesin proteins: Holds sister chromatids together at the centromere site

• Kinetochore: The protein/DNA complex at the centromere which serves as the site for microtubule attachment

• Centriole: Self replicating organelles in the cytoplasm of eukaryotic organisms (but not higher plants), that occur at right angles near the centrosome

• Centrosome: Organelles near the nucleus that serve as an animal's mitotic center used to assemble the spindle fibres

• Spindle Fibres are microtubules made of the protein tubulin

• They form during mitosis and meiosis to pull the chromosomes to the poles of a cell

Phases of Mitosis: Prophase

• Chromosomes condense making it easier to pull them apart later
• The mitotic spindle starts to form to organize chromosomes during mitosis and move them around
• As the centrosomes move apart, the spindle grows between them
• The nucleolus disappears
• The nuclear envelope breaks down releasing the chromosomes
• The mitotic spindle starts to “capture” chromosomes

Phases of Mitosis: Early Prophase

• The chromosomes start to condense
• The spindle is starting to form

Phases of Mitosis: Late Prophase (Prometaphase)

• The chromosomes become fully condensed

• The nuclear envelope breaks down

• Microtubules bind to chromosomes at the kinetochore (a protein patch on the centromere)

• Kinetochore microtubules are those that bind a chromosome

• Microtubules that aren't bound to kinetochores stabilize the spindle by grabbing onto microtubules from the opposite pole

• These extra microtubules extend from each centrosome, forming a structure known as an aster

Phases of Mitosis: Metaphase

• The spindle has captured every chromosome and lined them up down the middle of the cell
• All of aligned at what's known as the operator of the cell (a plane where the chromosomes line up)
• Each chromosome's kinetochores should be attached to microtubules from opposite spindle poles
• The cell checks if all the chromosomes are at the metaphase plate and if the kinetochores are attached
• This is known as the spindle checkpoint, and ensures that the sister chromatids will separate properly

Phases of Mitosis: Anaphase

• The sister chromatids separate and are pulled towards the opposite ends of the cell
• Each chromatid becomes its own chromosomes during this phase
• Microtubules not attached to chromosomes elongate and separate the poles, making the cell longer

Phases of Mitosis: Telophase

• The cell is nearly done dividing and is beginning to re-establish its normal structure as cytokinesis (division of cell contents) takes place
• The spindle is broken down into it's building blocks
• Nuclei begin to form, one from each chromosomes
• Nuclear membrane and nucleoli reappear
• Chromosomes begin to de-condense

Cytokinesis

• Cytokinesis is the division of the cytoplasm to form two new cells

• In animal cells:

• Cytokinesis is contractile

• The cell is pinched in two with filaments made of actin

• This pinch crease is known as the cleavage furrow

• In plant cells:

• Cells can't contract because the cell wall is too stiff

• Structure called the cell plate forms splitting the cell in two down the middle

• Microtubule spindle structures known as phragmoplasts carry vesicle derived from the Golgi apparatus to the cell plate

• At the end of cytokinesis two new cells are made, each with a complete set of of chromosomes, similar to that of the mother cell

Differences in Cytokinesis of Animal and Plant Cells

Meiosis

• Meiosis is the division process that creates haploid cells (one set of chromosomes) from diploid cells (two set of chromosomes)

• In humans:

• Happens when sperm and cells join in fertilization

• Haploid cells made via meiosis are sperm and eggs

• Chromosomes from both cells create a diploid

• Similar to mitosis

• It still needs to separate sister chromatids

• More complex task to separate homologous chromosomes (similar but non identical pairs of chromosomes from each parent)

• Does this to half the chromosome number in daughter cells

• There are two rounds of division in meiosis, resulting in four gametes from the starting cell

• In Meiosis 1 homologue pairs separate

• In Meiosis 2 sister chromatids separate

Phases of Meiosis 1: Before Meiosis

• A cell must go through interphase first
• Grows while copying the chromosomes and preparing for division

Phases of Meiosis 1: Prophase 1

• Chromosomes start to condense
• They pair up in a process called synapsis (crossing over)
• Places where the chromosome pairs up are referred to as chiasmata
• Chromosomes align to homologue partner so that they match up to corresponding positions along full length and exchange DNA material, resulting in bivalents or tetrads

Metaphase 1

• After crossing over, the spindle captures chromosomes and moves them to the metaphase plate
• Each chromosome connects to microtubules from just one pole of the spindle and the two homologues connect from opposite poles
• Homologue pairs line up, unlike in Mitosis, and the orientation of the pairs is random
• This allows for the random assortment and formation of gametes with different sets of chromosomes

Anaphase 1

• Homologues are pulled apart and away to opposite ends
• Sister chromatids of each chromosomes still remain connected

Telophase 1

• Chromosomes arrive at opposite poles
• In some organisms, the nuclear membrane re-forms and the chromosomes de-condense
• This is skipped in others, since cells continue into Meiosis 2
• Cytokinesis then forms two haploid daughter cells

Phases of Meiosis 2

• Cells move from meiosis 1 to meiosis 2 without copying their DNA
• Meiosis 2 is a shorter/simpler process, essentially similar to mitosis
• Cells that enter are cells produced in meiosis with one chromosome from each homologue pair and two sister chromatids which separate

Phases of Meiosis 2: Prophase 2

• Chromosomes condense and nuclear envelope breaks (if needed)
• Centrosomes move apart, form a spindle between them, and start to capture chromosomes

Phases of Meiosis 2: Metaphase 2

• Sister chromatids of each chromosome are captured via microtubules from opposite spindle poles
• Chromosomes line up individually along the metaphase plate

Phases of Meiosis 2: Anaphase 2

• Sister chromatids separate and are pulled towards opposite poles of the cell

Phases of Meiosis 2: Telophase 2

• Nuclear membranes form around each set of chromosomes and decondense

• Cytokinesis divides chromosomes, setting into new cells

• Four genetically different daughter cells are produced

• Each chromosome contains has only one chromatid

Significance of Meiosis

• Cells made during meiosis are haploid, but are not genetically identical
• Many genetic variable present in gametes that go via crossing exists due homologues crossing over
• There is a degree of randomness when chromosomes cross over
• Random alignment and segregation of homologous chromosome pairs at both metaphase and anaphase promotes varied gene pools in gametes

Differences Between Mitosis and Meiosis