Cell and Nuclear Division

Cell and Nuclear Division

• Cells divide for growth and repair.
• Mitosis and Meiosis occur in eukaryotic cells.

Cell Proliferation

• The process involves a cell growing and dividing, resulting in 2 daughter cells.
• Cell proliferation causes an exponential increase in the number of cells.
• Cell proliferation is a mechanism for tissue growth and repair.

Embryological Development

• Embryological development involves early embryo growth.

Plant Meristems

• Undifferentiated cells reproduce and differentiate to produce plate tissue and organs.

Tissue Replacement and Healing

• Dead and damaged tissues are replaced by cell proliferation.
• Skin cells are lost and replaced throughout an animal's life.

Cell Division

• A parent cell divides to produce 2 daughter cells.
• Cell division occurs in all living organisms.

Nuclear Division

• Nuclear division, unlike cell division, includes mitosis and meiosis in eukaryotic cells.
• Nuclear division must occur before cytokinesis, ensuring each daughter cell has a nucleus.

Mitosis

• Mitosis involves one parent cell, resulting in 2 genetically identical daughter cells.
• Mitosis occurs from diploid to diploid
• Cytokinesis occurs after mitosis, resulting in 2 genetically identical daughter cells.
• Mitosis is necessary for for growth and development, tissue repair, and asexual reproduction in unicellular eukaryotic organisms.
• The cell life of mitosis goes from growth to DNA replication, and then further growth.

Mitosis Phases

• Mitosis has four phases and involves division of the nucleus.

Prophase

• Chromatin condenses to form chromosomes which become visible under a light microscope.
• Chromosomes consist of two identical sister chromatids attached by a centromere.
• The nuclear membrane(or envelope) breaks down
• Centrioles move toward the poles, producing spindle fibres.
• Chromosomes attach to the spindle fibres at the centromeres.

Metaphase

• Chromosomes line up along the equator of the cell, attached to spindle fibres.

Anaphase

• Spindle fibres separate the sister chromatids at the centromere to form single-stranded chromosomes.
• Microtubule motors on the kinetochore, attached to centromeres, move chromosomes to opposite poles.

Telophase

• Chromosomes arrive at the poles of the cell.
• Nuclear membranes enclose the chromosomes at each pole.
• Chromosomes uncoil to form chromatin.
• Two genetically identical nuclei are produced.
• Cytokinesis begins during telophase.

Cytokinesis

• Cytokinesis splits the cytoplasm of a parent cell into two daughter cells.
• Cytokinesis differs in animal and plant cells.
• Cytokinesis occurs after nuclear division (mitosis or meiosis).

Animal Cell Cytokinesis specifics

• A contractile ring of actin and myosin filaments forms around the equator.
• Myosin and actin attach to the plasma membrane and contract to form a cleavage furrow.
• The cleavage furrow deepens until the two daughter cells separate.

Plant Cell Cytokinesis specifics

• The golgi apparatus produces vesicles of carbohydrates that line up along the equator.
• The vesicles fuse to form a cell plate.
• Membranes from the vesicles fuse with the existing plasma membrane on either side of the cell plate.
• Two separate cells form.
• Cellulose is secreted into the cell plate to form the cell wall.

Cytokinesis Division

• The division of the cytoplasm is typically equal (equal size).
• Each daughter cell must receive at least one mitochondrion, because mitochondria reproduce independently.
• A cell lacking mitochondria cannot produce replacement mitochondria.
• Other organelles can only be produced from a pre-existing structure and have their own DNA.

Interphase

• Majority of a cell's lifespan is spent in interphase.
• Interphase involves normal metabolic activity, protein synthesis, and reparation.

Phase G1

• The cell grows after mitosis and cytokinesis.
• Extensive protein synthesis and creation of new organelles occurs.

Phase S

• DNA replication occurs to produce chromosomes with sister chromatids.

Phase G2

• Cells prepare for mitosis by growing and replicating organelles.

DNA Replication

• DNA replication is a prerequisite for nuclear division (mitosis/meiosis).
• DNA replication produces 2 identical copies of a DNA molecule and a chromosome, held together by a centromere.
• Sister chromatids are two identical copies of a duplicated chromosome (maternal + parental).
• Homologous pairs are a pair of chromosomes (not identical).
• Only sister chromatids are identical.

Cyclins

• Hormones control cell cycle speed.
• Progress to the next cell cycle process depends on the concentration of specific cyclins.
• Cyclins bind to Cyclin-dependent Kinases (CDK).
• CDK is a group of enzymes.
• Cyclins bind to and activate a specific CDK enzyme.
• Activated enzymes attach to phosphate to specific proteins initiating a cascade of reactions that trigger events within the cell cycle.
• A cell cycle that's too fast causes can lead to tumours or cancer.
• Benign tumors are non-cancerous and don't spread.
• Malignant tumors are cancerous and spread.

Mutations of Cell Cycle Genes

• Mutations are changes to the DNA or RNA base sequence in a cell or virus.

Tumour Suppressor Genes

• Tumour suppressor genes cause regular cell division by inhibiting cell proliferation and tumour development.
• Loss of function in tumour suppressor genes results in uncontrolled cell division.

Proto-oncogenes

• Proto-oncogenes regulate normal cell growth and speed.
• Mutation of proto-oncogenes can cause it to become an oncogene.

Oncogene

• Oncogenes convert a normal cell into a tumour cell, leading to cancer.
• Oncogenes cause a rapid cell cycle rate.

Mitotic Index

• Mitotic Index is calculated by the number of cells undergoing mitosis / number of cells.
• A high mitotic index may indicate cancerous tissue.

Cancer

• Mutation of proto-oncogenes and tumour suppressor genes can lead to uncontrolled cell division.
• Cancer cells grow and divide more rapidly than normal.

Primary Tumours

• The first tumour produced in the body.
• Cancer cells from the primary tumour can spread to form secondary tumours.

Secondary Tumours

• Secondary tumours result from metastasis of cancer cells.

Benign Tumours

• Benign tumours are non-cancerous.
• Benign tumours grow slowly, don't invade neighbouring tissue, and don't undergo metastasis.

Malignant Tumours

• Malignant tumours are cancerous.
• Malignant tumours grow quickly and undergo metastasis.

Meiosis

• Meiosis is reduction division to create gametes, from 1 diploid to 4 haploids.
• Includes homologous pairs (2 chromatids)
• Homologous chromosomes (non-sister chromatids) pair up to form bivalents during meiosis I.
• Bivalents curl up over each other many times.
• Occurs due to crossing over between non-sister chromatids at the chiasma.
• Meiosis involves 2 nuclear divisions followed by cytokinesis (PMAT x2).

Meiosis I

• Meiosis I segregates homologous chromosomes, producing two haploid cells.

Meiosis II

• Segregates the sister chromatids producing 4 haploid cells.

Meiosis I - Prophase I

• Crossing over switches alleles between non-sister chromatids.
• Forming bivalents.
• Centrioles move towards the poles and create spindle fibres.

Meiosis I - Metaphase I

• Pairs are in the middle.
• Spindle fibres move bivalents (homologous chromosomes) to the equator.
• Sister chromatids attach to spindle fibres at the centromere.
• Independent assortment of each homologous chromosome pair.
• Maternal and paternal homologous chromosomes line up randomly.

Meiosis I - Anaphase I

• Pulled away by spindle fibres
• Homologous chromosomes of bivalents are separated and pulled towards the poles.

Meiosis I - Telophase I

• 2 newly formed nuclei.
• Reached opposite ends that uncoil.
• Form clear membrane around sister chromatids (non-identical) at each pole, producing haploid nuclei.
• Cytokinesis occurs, producing two haploid cells.

Meiosis II

Prophase II

• Condensing.
• Forms Spindle fibres
• No crossing over.

Metaphase II

• Gathers at the middle
• Single file line.

Anaphase II

• Chromatids that are pulled away by spindle fibres

Telophase II

• Nuclei reform
• 4 cells forming.

Results of Meiosis

• Haploids.

Issues With Meiosis

• If there are too many or too little chromosomes in a separation, genetic disorders can happen.
• Down syndrome is an example, along with miscarriages.
• These usually happen because of egg disjunctions.
• Down syndrome is caused by 3 copies of chromosome 21 aka trisomy 21

Meiosis Purpose

• Required for the production of gametes and spores.