all cells arise from other cells

• what is a chromosome?

  - one DNA molecule folded into a condensed form, wrapped around histone proteins in eukaryotes

  - following DNA replication, the chromosome is made of 2 genetically identical sister chromatids

  - sister chromatids are attached at the centromere
• what is a homologous pair?

  - one chromosome is inherited from the mother and the other is inherited from the father

  - chromosomes come in homologous pairs
• what is the definition of homologous chromosomes?

  have the same genes but different alleles
• what is a gene?


  section of DNA that codes for a specific polypeptide or function RNA
• what is an allele?


  different forms of the same gene that codes for a specific trait
• haploid and diploid cells


  - diploid cells (all body cells) = homologous pairs of chromosomes (2n)

  - haploid cells (gametes) = one copy of each homologous pair of chromosomes (n)
• main stages of the cell cycle

  - interphase

  - mitosis (prophase, metaphase, anaphase + telophase)

  - cytokinesis
• interphase

  - the cell grows + prepares for mitosis:

  -> G1 phase: cell organelles are replicated

  -> S phase: DNA replication

  -> G2 phase: increase in protein synthesis + ATP synthesis

  
  

  INDIVIDUAL CHROMOSOMES ARE NOT VISIBLE
• what is mitosis and its purpose?

  - cell division that produces 2 genetically identical daughter cells from a parent cell

  
  

  -> growth

  -> repair tissues

  -> replace cells

  
  

  - mitosis is a highly controlled process, controlled by certain genes
• prophase

  - chromosomes condense + become visible

  - nuclear membrane breaks down

  - centrioles move to opposite poles of the cell

  - spindle fibres form

  
• metaphase

  - spindle fibres attach to centromere

  - chromosomes are lined up at the equator of cell

  
• anaphase

  - spindle fibres contract, splitting the centromere

  - v shaped sister chromatids are separated + pulled apart to opposite poles of the cell

  
• telophase

  - identical chromosomes have reached the poles

  - spindle fibres break down

  - new nuclear membrane forms around each group of chromosomes

  - chromosomes un-condense + begin to form chromatin again

  
• cytokinesis

  - cytoplasm divides, producing 2 genetically, identical daughter cells

  
• what happens if a mutation occurs?

  - occur at random

  - causes uncontrolled cell division, which leads to the formation of tumours + cancers
• mutagenic agents


  - increase the chance of mutation

  - eg UV radiation, ionising radiations, chemicals from cigarettes
• cancerous cells

  - divide uncontrollably

  - divide more often

  - have a shorter cell cycle

  
  

  CELLS ARE UNSPECIALISED + HAVE NO FUNCTION
• what is cancer?

  presence of a malignant tumour
• types of tumours

  malignant:

  -> grows rapidly

  -> cells break off + spread to other parts of the body

  benign:

  -> slow-growing

  -> do not spread
• cancer treatments are aimed at controlling the rate of cell division. these drugs may stop:

  - prevent synthesis of enzymes needed for DNA replication

  - damage DNA

  - cytokinesis

  - prevent formation of spindle fibres
• problems with cancer treatments

  - treatments do not distinguish healthy cells from cancerous cells, so they also destroy normal body cells that are dividing, causing side effects

  - however, tumour cells divide more frequently than normal cells, so the treatments target tumour cells more
• binary fission

  prokaryotic cells replicate by a process called binary fission

  
  

  binary fission:

  - CIRCULAR DNA and plasmids REPLICATE

  - cell gets bigger + plasmids move to opposite poles

  - CYTOPLASM DIVIDES to produce 2 DAUGHTER CELLS

  - EACH CELL contains a SINGLE COPY OF CIRCULAR DNA but a variable number of plasmids
• viral replication

  - attachment proteins bind to receptors

  - viral nucleic acids (DNA/RNA) enters cell

  - nucleic acid replicated in cell

  - cell produces viral proteins eg a capsid

  - virus assembled and released from cell -> this can cause cell lysis