topic 3

• living organisms 


  all living organisms are made of cells and share some common features

  in multicellular organisms, cells are organised into tissues, tissues into organs and organs into organ systems
• nucleus 


  Surrounded by envelope (double membrane) which contains pores enabling molecules to enter and leave the nucleus

  the nucleus contains DNA wrapped around histone proteins in a complex called chromatin, and a nucleolus which is the site of ribosome production
• Rough endoplasmic reticulum 


  a series of flattened sacs enclosed by a membrane with ribosomes on the surface

  The RER folds and processes proteins made on the ribosomes; often located close to nucleus
• smooth endoplasmic reticulum


  a system of membrane-bound sacs

  the SER produces and processes lipids
• Golgi apparatus 


  series of fluid-filled, flattened and curved sacs with vesicles surrounding the edges.

  The Golgi apparatus modifies and packages proteins (after budding off the RER) + lipids. It also produces lysosomes
• mitochondria 


  usually oval shaped and bound by a double membrane called the envelope.

  the inner membrane is folded to form projections called Cristae, with a fluid matrix on the inside containing the enzymes needed for cellular respiration
• centrioles 


  hollow cylinders containing a ring microtubules arranged at right angles to each other

  Centrioles are involved in cell divisions
• 80S ribosomes 


  composed of 2 subunits.

  The site of protein synthesis
• lysosomes 


  vesicles, containing digestive enzymes, bound by a single membrane
• structure of a eukaryotic cell


  :

  
• protein trafficking

  • proteins produced on the ribosomes on the surface of the RER
  • these are then folded and processed in the RER
  • the proteins then transported from RER to Golgi apparatus in vesicles
  • they are modified in Golgi apparatus
  • The Golgi apparatus packages proteins into vesicles to be transported around cells where required. Some proteins - extracellular enzymes leave cell by exoccytosis
• structure of prokaryotic cell

  :

  
• cell wall


  cell's rigid outer covering made of peptidoglycan

  Provides cell with strength and support
• slime capsule 


  protective slimy layer which helps cell to retain moisture and adhere to surfaces
• plasmid 


  circular pieces of DNA
• flagellum


  tail-like structure which rotates to move the cell
• pilli


  hair-like structures which attach to other bacterial cells (allowing exchange of plasmids)
• 70s Ribosomes 


  composed of 2 subunits

  The site protein synthesis
• Mesosomes 


  infoldings of the inner membrane - their function is debated with many scientists believing they are just artefacts from the preparation process for microscopy, others believe they contain enzymes required for respiration
• ovum (egg)

  structure :

  
• zona pellucida 


  a protective coating that the sperm have to penetrate in order for fertilisation to occur, it hardens after they have entered

  The main purpose of the zona pellucid is to prevent polyspermy
• haploid nucleus

  so that full set of chromosomes is restored at fertilisation
• cortical granules 


  release substances which causes zona pellucida to harden
• follicle cells 


  form a protective coating around the egg
• spermatozoa (sperm)


  structure:

  
• how sperm is adapted for its function

  contain many mitochondria to provide energy for rotation of the flagellum which enables the cell to move

  acrosomes contain digestive enzymes which break down the zona pellucida and allow sperm to penetrate the egg
• fertilisation

  1. sperm head meets zona pellucida and acrosome reaction occurs - enzymes digest zona pellucida
  2. sperm head fuses with the cell membrane of the egg cell allowing sperm nucleus to enter egg cell
  3. cortical reaction occurs which causes the zona pellucida to harden preventing polyspermy
  4. nuclei fuse and full set of chromosomes is restored, forming diploid zygote
• locus

  location of genes on a chromosome
• linkage of genes on a chromosome 


  alleles on the same chromosome can be autosomal linked and are inherited as if they are the same gene

  the closer the loci of genes on the chromosome, the more closely linked they are

  this is because they are less likely to be separated during recombination in meiosis
• sex-linkage 


  some genes are sex linked because they occur on the x chromosome

  some genetic disorders are sex linked and therefore much more common in men. This occurs because the Y chromosome is smaller than the x chromosome, so if men don't have another copy of the allele for specific characteristic.

  as result they only need one copy for alleles to be expressed

  e.g haemophilia (absence of clotting factor) or red-green colourblindness
• cell cycle 


  process in which cells divide to produce two genetically identical daughter cells fro growth, repair and asexual reproduction.

  As cell produced by mitosis are genetically identical, it doesn't give rise to genetic variation

  There are 3 stages
• stages of cell cycle 


  1. mitosis- prophase, metaphase, anaphase and telophase
  2. cytokinesis
  3. interphase
• prophase

  chromosomes condos and are visible when stained

  chromosomes: 2 identical sister chromatids joined at centromere

  2 centromeres move to opposite poles

  spindle fibres emerge from centrosomes

  nuclear envelope breaks down into small vesicles

  
• metaphase

  centrosomes reach opposite poles

  Spindle fibres extend from chromosomes

  chromosomes line up at equator

  spindle fibres reach chromosomes and attach to centromeres

  each sister chromatid attaches to spend fibre originating from opposite poles

  
• anaphase 


  sister chromatids separate at centromere

  spindle fibres begin to shorten

  chromosome (separated sister chromatids) pulled to opposite poles by spindle fibres

  
• telophase 


  chromosomes arrive at opposite poles and begin to decondense

  nuclear envelopes begin to reform around each seton chromosomes

  spindle fibres break down

  
• cytokinesis 


  cytoplasm divides producing 2 daughter cells
• interphase 


  G1 - cell grows and DNA replicates

  S - chromosomes replicated and begin to condense to form chromatin

  G2 - cel prepares to divide, replicating organelles for full set in each new cell
• meiosis 


  form of cell division that gives rise to genetic variation

  • main role of meiosis is production of haploid gametes and maintenance of chromosome number as cells produced by meiosis have half the number of chromosomes
  • meiosis produces genetically different cell, genetic variation is achieved crossing over, independent assortment
• crossing over 


  the exchange of sections of DNA between homologous chromosomes