2.1.6 Cell Division

• Cell Cycle in Prokaryotes

  Cell grows, DNA replicates, cell divides
• Eukaryotic Cell Cycle

  Several phases; Interphase, Mitosis, Cytokinesis
• Interphase

  Three phases within interphase: G1 phase, S phase, G2 phase
• G1 Phase

  Cell grows rapidly while performing routine metabolic processes. Makes proteins needed for DNA replication and copies some of its organelles in preparation for cell division. Cell spends most of its life in this phase.
• G1 Checkpoint

  Checks for cell size, nutrient growth factors and DNA damage.
• S Phase

  Cell's DNA is copies in the process of DNA replication
• G2 Checkpoint

  Checks for cell size, DNA damage and replication
• G2 Phase

  Cell makes final preparations to divide; makes additional proteins and organelles. Cell continues to grow, energy stores increase, duplicated DNA is checked for errors, preparing for mitosis.
• Metaphase/spindle assembly checkpoint

  Checks for chromosome attachment to spindle fibres; mitotic checkpoint
• Mitosis

  Division of chromosomes into two genetically identical sets.
• Prophase

  Nucleolus fades, chromosomes duplicate and move to opposite poles of the cell; centrioles form spindle fibres; chromosomes condense and thicken, becoming visible, two chromatids are visibly distinct; nuclear envelope disintegrates

  
• Metaphase

  Chromosomes line up along the metaphase plate; one chromatid of each chromosome lies on either side of the cell; spindle fibres attach to the centromere

  
• Anaphase

  The spindle fibres being to contract; sister chromatids are pulled apart and move to opposite poles of the cell, centromere first; centromere divides

  
• Telophase

  Seperation of chromosomes is complete as they reach the poles of the cell and uncoil; each chromatid is now a daughter chromosome; spindle fibres disintegrate; nucleolus is formed; nuclear envelop reforms around each group of daughter chromosomes.

  
• Cytokinesis

  Separation of cytoplasm to form two genetically identical daughter cells.

  
• Chromosome

  Coiled structure of DNA and protein that is the form of the genetic material of a cell during cell division.
• Chromatid

  Two DNA strands joined together by their centromere
• Homologous chromosome

  Chromosome pair; 23 of these in a human cell
• Meiosis

  Division of a diploid nucleus to form four genetically different haploid daughter nuclei.
• Meiosis 1

  Separating homologous chromosomes to create two haploid cells; reduction division
• Prophase 1

  Nucleolus fades; centrosomes duplicate and move to opposite poles of the cell; centrioles form spindle fibres which attach to the centromere; chromosomes condense and thicken, becoming visible, two chromatids are distinct; nuclear envelope disintegrates; homologous chromosomes pair up, forming bivalents; crossing over occurs.
• Metaphase 1

  Homologous pairs line up at the metaphase plate/equator; each member of each homologous pair could be on either side of the equator - known as independent assortment.
• Anaphase 1

  Spindle fibres begin to shorter; homologous chromsomes are pulled apart and to opposite poles of the cell, centromere first; the member of the homologous pair that goes to each pole depends on the line up in metaphase 1; centromeres do not divide, chromatids remain paired.
• Telophase 1

  Separation of homologous pairs is complete; reach poles of the cell and uncoil; nuclear envelope reforms around each group of chromosomes; each new nucleus has half the original number of chromosomes, each chromosome consists of two chromatides.
• Cytokinesis

  Cell divides to form two daughter cells; can be followed by short resting phase of interkinesis.
• Meiosis 2

  Separating sister chromatids to form four genetically different haploid daughter cells.
• Prophase 2

  Centrioles form spindle fibres which attach to centrosomes; chromosomes attach and coil, each chromosomes consists of two chromatids which are no longer genetically identical due to crossing over in prophase 1; if reformed, nuclear envelop disintergrates again.
• Metaphase 2

  Chromsomes line up at the metaphase plate; each sister chromatid of each chromosome could be on either side of the equator (independent assortment) - determines which daughter cell each chromatid ends up in.
• Anaphase 2

  Spindle fibres begin to contract; sister chromatids pulled apart and move to opposite poles of the cell, centromere first; centrosomes divide; which chromatid ends up where depends on line up in metaphase 2.
• Telophase 2

  Separation of sister chromatids is complete as they reach poles of the cell and reform; nuclear envelope reforms around each group of chromosomes; each new nucleus has half the original number of chromosomes, each chromosome consists of one chromatid
• Cytokinesis

  Two cells divide to form four haploid, genetically different daughter cells. In plants, a tetrad of four daughter cells is formed.
• Crossing Over

  Happens in prophase 1; the exchange of alleles between homologous chromosome. Bivalent is where they line up, they cross at a chiasma. Each of the four daughter cells of meiosis contain chromosomes with different combinations of alleles; hence genetically different.
• Independent Assortment

  Metaphase 1 and 2. In metaphase 1, homologous pairs line up on the equator, which side the maternal and paternal chromosome is on is random. In metaphase 2 the chromsomes line up on the equator and the sister chromatids split. Contains a random combination of maternal and paternal alleles which leads to genetic variation.
• Random fertilisation

  Occurs during sexual reproduction; two gametes fuse together. Which gamete duses with which is random, introducing genetic variation as each gamete is genetically different.
• Stem Cells

  Unspecialised cells that are characterised by their ability to undergo cycles of self renewal as well as being able to differentiate into a specialised cell type. THey are able to express all their genes and divide by mitosis.
• Differentiation

  The process during development whereby newly formed cells become more specialised and distinct from one another as they mature. Unnecessary genes are turned off, organelle content and distribution change and shape and location of cell changes.
• Totipotent

  Can produce a whole new organism, in animal cells it can form the embryo, placenta and umbilical cord. Cell that can differentiate into any cell type in the body.
• Pluripotent

  Blastocyst (cluster of cell) that has the potential to differentiate into any cell type in the body but not whole organisms; they are embryonic stem cells.
• Multipotent

  Limited number of cell types that they can differentiate into - they can only form the cell types that make up the tissue in which they are found, e.g. adult stem cells.
• Unipotent

  Can only differentiate into one cell type; have the property of self renewal which distinguishes them from non-stem cells.