biology topic 3

• what are the common features of a eukaryotic cell?

  linear genetic material contained in a nucleus

  membrane bound organelles

  larger ribosomes (80S) than prokaryotes (70S)
• what are the common features of a prokaryotic cell?

  no nucleus or membrane bound organelles

  has mesosomes not mitochondria

  DNA is circular
• describe the structure and role of a mitochondrion

  A double membrane bound organelle which is the site of aerobic respiration. The inner membrane is folded to form cristae.
• Describe the role of the centriole

  Involved in the formation of a spindle in cell division; in animal cells.
• Describe the structure and role of the nucleus

  Contains nucleolus and chromosomes - chromosomes are made of DNA and contain genes which control protein synthesis.
• Describe the structure and role of a nucleolus

  A dense body within the nucleus where ribosomes are made.
• Describe the structure and role of ribosomes

  The site of protein synthesis; found free in the cytoplasm or attached to the Rough Endoplasmic Reticulum - made of rRNA and protein.
• Describe the structure and role of the rough Endoplasmic Reticulum

  A system of interconnected membrane-bound flattened sacs, with ribosomes attached to the surface.
• Describe the structure and role of lysosomes

  Membrane-bound spherical sacs containing digestive enzymes, which are involved in the breakdown of unwanted cell structures.
• Describe the structure and role of smooth endoplasmic reticulum

  A system of interconnected membrane-bound, flattened sacs which make lipids.
• Describe the structure and role of the Golgi apparatus

  Stacks of membrane-bound flattened sacs which modify and package proteins.
• Describe the structure of a sperm cell

  Head piece contains haploid nucleus and acrosome which digest through the zona pellucida. Midpiece has lots of mitochondria for swimming and there is a flagellum.
• Describe the structure of an ovum

  Haploid nucleus - 23 chromosomes

  Cortical granules - for the cortical reaction

  Zona Pellucida - prevents polyspermy

  Lipid rich cytoplasm - energy source for a zygote
• Describe Protein Trafficking

  Transcription of mRNA

  Packaging into a vesicle

  Fusion with the Golgi apparatus

  Modification of the protein

  Transfer to the secretory vesicles

  Pinched off the Golgi apparatus

  Exocytosis to release from the cell
• Acrosome Reaction

  Cells surrounding the ovum release chemicals triggering the acrosome reaction

  Acrosome swells and fuses with the cell surface membrane

  Acrosome releases digestive enzymes

  Enzymes digest the follicles of the zona pellucida
• Cortical reaction

  Lysosomes release digestive enzymes via exocytosis

  the zona pellucida thickens

  polyspermy is prevented
• Diploid cell 


  46 chromosomes in a nucleus
• Haploid cell

  23 chromosomes in a nucleus
• Describe meiosis

  Crossing over allows chromosome sections to exchange at chiasma

  Independent Assortment occurs where chromosomes are randomised and placed into 4 individual gametes
• Describe linkage

  The closer together two genes are on a chromosome, the more likely they will be inherited together.
• Homologous pair

  A matched pair of chromosomes which have the same genes in the same loci, alleles may differ.
• Chromatid


  Half of a chromosome
• Sex Linkage 


  A trait carried on the female chromosome, females are carriers and men are sufferers.
• Describe the Cell Cycle

  Interphase - split into G1: protein synthesis, S: DNA replication, G2: protein synthesis and organelle replication

  Mitosis or Meiosis

  Cytokinesis
• Cytoplasmic division in animal cells

  Cell surface membrane constricts around the cell centre and a ring of protein filaments bound to the inside surface contract until the cell is divided into 2.
• Cytoplasmic division in plant cells

  A cell plate is synthesised between the 2 cells
• Mitosis

  Prophase: chromosomes condense and become visible, the centriole begins to produce spindle fibres and the nuclear envelope breaks down

  Metaphase: spindle attaches to centromere and moves the chromosomes to cell equator

  Anaphase: centromere splits, spindle fibres shorten and sister chromatids are pulled to opposite poles of the cell

  Telophase: 2 new nuclear envelopes appear and chromosomes uncoil
• Stem cells

  An undifferentiated cell which can divide by mitosis and become differentiated
• Totipotent 


  A stem cell that can differentiate into any cell type
• Multipotent

  A stem cell that can differentiate into some cell types
• Pluripotent


  A stem cell which can differentiate into most cell types
• Gene expression

  A stimulus activates transcription factors

  Transcription factors bind to a promoter region

  A transcription initiation complex is formed

  RNA polymerase can bind resulting in transcription and translation

  THE GENE IS SWITCHED ON
• DNA methylation

  A methyl group attaches to DNA (usually cytosine) preventing transcription by stopping the RNA polymerase binding so the gene is switched off.
• Histone modification

  Methyl or acetyl groups attach to histones affecting how tightly they are wound.
• Genotype 


  The genetic makeup of an organism.
• Phenotype 


  The physical expression of a gene which is a result of the genotype interacting with the environment
• Continuous variation 


  A characteristic which can take any value within a range - it can be multifactorial
• Discontinuous variation


  A characteristic which can only appear within specific discrete values
• Multifactorial


  Several genes and one or more environmental factors cause a condition
• Describe how the cell detecting UV produces melanin

  Melanosomes gather around the nucleus to protect the DNA from UV

  Melanin production is stimulated by melanin stimulating hormone

  As exposure to UV light increases, the amount of cell surface receptors triggered increases

  The amount of MSH increases so more melanin is made