L1 - the cell cycle and mitosis

• Three stages in the cell cycle:

  -interphase

  -mitosis

  -cytokinesis
• The cell cycle

  
• Cell cycle is controlled by checkpoints:

  -spindle assembly checkpoint

  -G1 checkpoint

  -G2 checkpoint
• Spindle assembly checkpoint:

  -checks for chromosome attachment to spindle fibres in mitosis
• G1 checkpoint checks for:

  -cell size

  -nutrients

  -growth factors

  -DNA damage
• G2 checkpoint checks for:

  -cell size

  -DNA replication

  -DNA damage
• What is G0:

  -when the cell leaves the cell cycle

  -this may be a temporary resting period or more permanent. An example of the latter is a cell that has reached an end stage of development and will no longer divide (eg. neuron)
• A cell may reach a more permanent G0 stage if it:

  -has become differentiated (specialised)

  -has damaged DNA

  -has become senescent
• Mitosis specifically refers to the process of nuclear division that occurs before a cell physically divides in two
• During mitosis:

  -the cells DNA is copied into each of the two daughter cells. in multicellular organisms, mitosis provides new cells for growth and tissue repair

  -in eukaryotes, it can also be a form of asexual reproduction. this most commonly occurs in single celled organisms such as yeast (where its called budding)
• Importance of mitosis:

  -formation of two new identical daughter cells from an original

  -how organisms grow

  -basis of simple asexual reproduction

  -in diploids there are two copies of each chromosome

  -in human body there are 23 pairs of chromosomes making a total of 46
• Mitosis is a continuous sequence but is divided into four stages:

  -prophase

  -metaphase

  -anaphase

  -telophase
• In mitosis, each chromatid contains identical genetic information, so each daughter cell also contains identical information
• Chromosomes structure:

  -chromosomes are only visible during cell division

  -each chromosome consisted of two chromatids joined somewhere along its length at the centromere

  -genetic information (genes/alleles) carried on each chromatid is identical
• Replicating chromosomes:

  
• Early prophase:

  -chromosomes become more distinct

  -chromosomes coil up, shorten, thicken, take up stain more intensely

  -centriole divides

  -nucleolus becomes less prominent

  
• Late prophase:

  -chromosomes become more distinct and are seen to consist of two chromatids joined by a centromere

  -centrioles migrate to opposite poles of the cell

  -nucleolus continues to shrink and disappears

  -nuclear envelope disintegrates

  -protein microtubules form spindle-shaped structures linking the poles to the cells, the spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell

  
• The spindle fibres are necessary to move the chromosomes into the correct positions before division
• In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell during prophase. The centrioles are cylindrical bundles of proteins that help in the formation of the spindle
• Metaphase:

  -each centriole is at a pole

  -centrioles grow/produce spindle fibres

  -spindle fibres attach to the centromere of the chromosomes

  -each centromere is attached to both poles

  -chromosomes pulled to the metaphase plate or equator

  
• Anaphase:

  -spindle fibres contract

  -the centromere divides

  -chromatids (daughter chromosomes) are pulled to opposite poles of the cell

  -centromere is pulled first creating a V shape

  -each half of the cell receives one chromatid from each chromosome

  
• Telophase:

  -chromatids reach the poles of the spindle

  -chromatids begin to uncoil

  -chromatids become less distinct

  -chromatids are known as daughter chromosomes once they reach the poles of the spindle

  
• When the chromosomes begin to uncoil during telophase, the nucleolus is formed. Cell division (cytokinesis) now begins
• Cytokinesis (after mitosis):

  -the cell divides

  -in animal cells this begins by constriction from the edges of the cell

  -in plant cells a cell wall is laid down

  -daughter cells have the same chromosome number and genetic makeup as each other and the parent cell
• In animal cell cytokinesis, the cytoskeleton pulls the membrane inwards
• In plant cell cytokinesis, a new cell wall is formed by vesicles and developing cell plates
• Cell division in yeast:

  -yeast is a fungus and well known for its role in bread and beer making

  -it reproduces asexually by a process called budding
• Budding:

  -nucleus divides but the chromosomes and spindle appear inside the envelope

  -envelope divides and migrates together with organelles into a bud that develops next to the site of nuclear division

  -bud then separates from the parent