2.1.6 - cell division, diversity, differentiation

• The cell cycle is a regulated cycle of division with intermediate growth periods.
• The stages of the cell cycle is

  interphase

  mitosis or meiosis (nuclear division)

  cytokinesis
• During interphase there are three main stages

  G1

  S

  G2
• In G1: cell synthesises proteins for replication (eg, tubulin for spindle fibres). Cell size doubles
• In G2: organelles divide
• Mitosis produces 2 genetically identical daughter cells for

  growth

  cell replacement /tissue repair

  asexual reproduction
• The four stages of mitosis are

  prophase

  metaphase

  anaphase

  telophase
• during prophase:

  1. chromosomes condense, becoming visible (x shaped - 2 sister chromatids joined at centromere)
  2. centrioles move to opposite poles of cell and mitotic spindle fibres form
  3. nucLear envelope and nucleolus break down so chromosomes are free in cytoplasm
• During metaphase:

  sister chromatids line up at the cell equator along the metaphase plate attached to the mitotic spindle by their centromeres
• During anaphase (which requires energy from ATP hydrolysis):

  1. spindle fibres contract = centromeres divide
  2. sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of cell with centromeres leading
  3. spindle fibres break down
• During telophase:

  1. chromosomes decondense becoming invisible again
  2. new nuclear envelopes form around each set of chromosomes = 2 new nuclei each with one copy of each chromosome
• During cytokinesis

  cell membrane cleavage furrow forms

  contractile division of cytoplasm
• Cell cycle is regulated by checkpoints which are regulated by cell-signalling proteins ensuring damaged cells do not progress to the next stage of the cycle
• at the checkpoint between G1 and S, the cell checks for DNA damage (eg via action of p53). After restriction point, cell enters cycle.
• Checkpoint between G2 and M the cell checks chromosome replication
• At metaphase checkpoint, cell checks that sister chromatids have attached to spindle correctly
• During meiosis 1:

  1. homologous chromosomes pair to form bivalents
  2. crossing over (exchange of sections of genetic material) occurs at chiasmata
  3. cell divides into two. Homologous chromosomes separate randomly. Each cell contains either paternal or maternal copy.
• Homologous chromosomes are a pair of chromosomes with genes in the same places. 1 maternal and 1 paternal. Some alleles may be he same while others are different.
• During meiosis 2:

  independent segregation of sister chromatids.

  Each cell divides again producing 4 haploid cells.
• Meiosis produces genetic variation by crossing over during meiosis 1. Independent assortment of homologous chromosomes and sister chromatids. Results in a new combination of alleles.
• A stem cell is an undifferentiated cell that can divide indefinitely and turn into other specific cell types.
• The four types of stem cell are

  totipotent

  pluripotent

  multipotent

  unipotent
• Totipotent stem cells can develop into any cell type including the placenta and embryo (early embryonic stem cells)
• Multipotent stem cells can differentiate into a few types of cell
• Unipotent stem cells can only develop into one type of cell.
• Cells become specialised as some genes are expressed while others are silenced due to cell differentiation. Cells produce proteins that determine their structure and function
• a transcription factor is a protein that controls the transcription of genes so that only parts of the DNA are expressed
• Transcription factors work by moving from the cytoplasm into the nucleus. Binding to promoter region upstream of target gene. Making it easier or more difficult for RNA polymerase to bind to gene. which increases / decreases rate of transcription
• The 2 groups of specialised cells in blood are erythrocytes and leucocytes
• Erythrocytes are red blood cells. They are biconcave so have no room for a nucleus but have lots of haemoglobin to carry oxygen around the blood
• Leucocytes are white blood cells. they can be lymphocytes, eosinophils, neutrophils which engulf foreign material
• Specialised cells in blood form from multipotent stem cells in the bone marrow. These differentiate into either erythrocytes - which have a short life span and cannot undergo mitosis. Or leucocytes including neutrophils
• The relationship between a system and specialised cells is:

  specialised cells -> tissues for specific functions -> organs made of several tissue types -> organ systems
• Simple squamous epithelium is a single smooth layer of squamous cells (thin and flat with round nucleus) fixed in place by a basement membrane
• Ciliated epithelium are made of ciliated epithelial cells which are column shaped with surface projections called cilia that move in a synchronised pattern
• Spermatozoon is specialised to fertilise an ovum during sexual reproduction in mammals.

  they have a haploid nucleus so fertilisation restores the diploid chromosome number

  acrosome secretes enzymes to penetrate ovum

  spiral shaped mitochondrion

  flagellum bound by plasma membrane to propel cell
• Root hair cells are specialised to absorb water and low concentration minerals from soil.

  They have hair-like projections which increase surface area for osmosis or carrier proteins for active transport.

  They have Many mitochondria produce ATP for active transport
• Meristems are totipotent undifferentiated plant cells that can develop into various types of plant cell including xylem and phloem.

  They are classified as apical (root and shoot tips), intercalary (stem) or lateral (vascular areas)
• A vascular bundle has the cambium (meristematic tissue) with phloem tissue and xylem tissue located in tubes around the cambium
• Xylem has:

  vessel elements which are lignified secondary walls for mechanical strength and waterproofing; perforated end walls for rapid water flow.

  tracheids which are tapered ends for close packing; pits for lateral water movement; no cytoplasm or nucleus