| what are gametes? | reproductive cells of an organism |
| two types of mammalian gametes | egg cell
sperm cell |
| features of egg cell | zona pellucida
haploid nucleus
cortical granules
follicle cells |
| zona pellucida | protective outer layer of the egg that sperm need to penetrate in order for fertilisation to occur
main purpose is to prevent polyspermy |
| haploid nucleus | contains half the set of chromosomes |
| cortical granules | contains enzymes which are released when a sperm and egg nuclei fuse to prevent polyspermy
enzymes harden the zona pellucida |
| follicle cells | forms a protective coat around the egg |
| features of sperm | flagellum
acrosome
mitochondria
haploid nucleus |
| flagellum | allows the sperm to move and swim to the egg |
| acrosome | contains digestive enzymes
the enzymes are released to digest the zona pellucida |
| mitochondria | provides energy to the flagellum so that the sperm can swim to the egg |
| outline the process of fertilisation | sperm swims to the egg
acrosome reaction - sperm membrane and acrosome membrane fuse, digestive enzymes released
enzymes digest and break down the zona pellucida
sperm and egg membrane fuses
sperm releases it nucleus and both nuclei fuse together making diploid nucleus
chemical signals from the egg trigger the cortical reaction
cortical granules release their enzymes by exocytosis
the enzymes harden the zona pellucida preventing polyspermy |
| what is a locus? | the location of genes on a chromosome |
| how can genes be linked? | must be on the same chromosome
small distance between loci therefore less likely to be split up due to crossing over and independent assortment |
| two types of cell division | mitosis
meiosis |
| meiosis | cell cycle that only occurs in gametes
produces four genetically different haploid daughter cells |
| how does meiosis achieve genetic variation? | independent assortment
crossing over |
| how does independent assortment help achieve genetic variation? | each gamete has one of many different combinations of chromosomes |
| what is crossing over? | the exchange of sections of DNA between homologous chromosomes |
| how does crossing over help achieve genetic variation? | results in recombination of genes increasing genetic variation |
| cell division cycle | has 3 stages: mitosis, cytokinesis and interphase
occurs in all cells except the gametes
produces 2 genetically identical diploid daughter cells
important for growth, repair and asexual reproduction |
| stages of mitosis | prophase
metaphase
anaphase
telophase |
| prophase | first stage of mitosis
nuclear membrane condenses and chromosomes are exposed |
| metaphase | second stage of mitosis
chromosomes line up at the centre of the cell |
| anaphase | third stage of mitosis
spindle fibres pull apart the sister chromatids to opposite ends of the cell |
| telophase | last stage of mitosis
nuclear membrane forms around chromosomes |
| root tip squash experiment | leave root tip in HCL to break up tissues and separate the cells so mitosis can be observed
stain cells with toluidine blue to see mitosis occur better
observe mitosis through microscope
calculate mitotic index by counting the no. of cells undergoing mitosis dividing by the total number of cells present |
| homologous chromosomes | chromosomes that are similar but not identical |
| sister chromatids | chromosomes that are identical |
