occurs in reproductive organs and results in the formation of sex cells Meiosis forms the ovum and sperms in animals.
Key Haploid gametes (23) Haploid Where does Egg (23) Diploid meiosis occur in a male and a female human? Sperm (23) MEIOSIS FERTILISATIONOvary Testis Diploid zygote (46) Mitosis and Testes in development males & Multicellular diploid ovaries in adults (46) females
MALE FEMALESex organs Testes OvariesGametes Sperm Ovum / egg
First division Second divisionFOUR daughter cells
Genetic variation isalmost infinite as a result of meiosis
First division Seconddivision NEW COMBINATIONS
1. exchange of genetic material during meiosis2. random fertilisation
Random Fertilisation = even more variation A human ovum plus a human sperm - 1 of 8 million combinations possible for each ovum and sperm - 223 x 223 = over 70 trillion combinations Any 2 parents will produce a zygote with over 70 trillion possible combinations without considering crossing over UNIQUE!!
Mitosis Meiosis1) Two cells are produced Four cells are produced2) Cells have the diploid Cells have the haploid number of chromosomes number of chromosomes3) One division occurs Two divisions occur4) Cells are genetically Cells are geneticallyidentical (CLONES) CLONES different5) Produces somatic cells Produces sex cells
46 Write the 23 23 numbers ofchromosomes found in the cells shown. 46 46
are thread-like structures found in the nucleusthey only show up clearly at cell division as they shorten and thicken
Number of ChromosomesFruit Flies: 8 Carrots: 20 Dog: 48 Shrimp: 254 Adders tongue (Fern): 1260 Largest number of chromosomes
The diagram shows part of a DNA molecule. Complete thediagram by labelling the bases. T G G A
is a series of chemical structures called bases along the length of a chromosomeprovides ‘instructions’ to the cell, e.g. to make pepsin or the skin colour
is a spontaneous change in a gene or a chromosome which may produce a change in the characteristic under its controla mutation can be: harmful lethal beneficial (mutations in bacteria often produce resistance to drugs)
Naturally when chemicals on the DNA change position during nuclear divisionArtificially by means of mutagens
an agent that causes a change in genetic composition
Question: MAY, 2004 (pg 78)It is not advisable for pregnant females to take X-rays because these can cause mutations in thedeveloping foetus. The X-rays can in fact act as amutagen (or mutagenic agent).ii)Explain the term mutation by referring to theDNA molecule. (2)A spontaneous change in the structure of DNA.A change in the sequence of bases occurs.
Question: pg 78ii) Name one human disease or condition which results from a mutation. (1)Haemophilia / Down’s syndromeiii)Name one mutagen other than X-rays. (1)UV rays