Meiosis

• Describe meiosis. 

• Reduction division

•  Parent cell (diploid nucleus) divides to produce different four cells (haploid nuclei)

• Describe the importance of meiosis in sexual reproduction. Creates sex cells ( gametes) 

• Describe haploid. One copy of every chromosome

•  gametes

• Ex: sperm or egg

• Describe diploid. Two copies of every chromosome

• Ex:  Somatic cells (body)

• Describe homologous chromosomes. chromosomes with same structural features and genes

• genes are the same,  alleles may be different. 

• Describe tetrad/bivalent. One pair of chromosomes (homologous chromosomes) in a tetrad. A tetrad is the association of a pair of homologous chromosomes (4 sister chromatids) physically held together by at least one DNA crossover.

• Describe crossing over. Non-sister chromatids swap alleles.

• Outline all the different phases of meiosis (1 mark each)

Prophase I 

• Synapsis:  homologous chromosomes form “tetrad”/bivalents

• Crossing over:  non-sister chromatids swap alleles

•  may form new allele combinations

• “recombinants”

• Increases genetic variation

       Metaphase I

• homologous pairs “tetrads/bivalents” line up in middle of cell 

       Anaphase I: homologous chromosomes separate to opposite poles

• Nondisjunction can occur

       Telophase/cytokinesis I

• Cell splits into 2 haploid cells with dyad chromosomes

      Interkinesis 

• pause between meiosis I and meiosis II 

•  proteins are made

•  No replication of chromosomes (as would be seen in interphase)

      Prophase II: Dyad chromosomes visible

       Metaphase II: Dyad chromosomes in middle

       Anaphase II: dyad chromosomes separate and sisters chromatids (monad) move to opposite poles

• Nondisjunction can occur

       Telophase II/Cytokinesis II: Each cell splits in two (4 haploid cells with monad chromosomes)

• Distinguish between interphase and interkinesis

• Outline the causes of genetic variation in humans. 

• Prophase I:  crossing over

• Metaphase I:  homologous chromosomes line up randomly

• law of independent assortment

• Anaphase I:  homologous chromosomes are separate randomly

• Human haploid number:  23 chromosomes=over 8 million gamete combinations (2^23)

• Other sources of genetic variation:  random fertilizations, DNA mutations, chromosome mutations, nondisjunction

• State the two phases a non disjunction can occur in. 

• Anaphase I-homologous chromosomes fail to separate 

• Anaphase II- dyad chromosome fails to separate

• Describe the causes of down syndrome. 

• Person has three copies of chromosome 21(trisomy 21)

• Generally, egg had 2 copies of chromosome 21 (nondisjunction)

• More likely to happen with an increase in  

• Sperm had 1 copy of  chromosome 21

• Be able to look at a karyogram and identify if trisomy 21 has occurred.  Also be able to determine the sex of the person

• State the correlation between mother’s egg age and the incidence of chromosomal abnormalities. A strong positive correlation exists between a mother's egg age and the incidence of chromosomal abnormalities, meaning that as a woman's eggs get older, the likelihood of her child having a chromosomal abnormality increases significantly. 

• Compare Mitosis and Meiosis.

AHL Material: HL students only:

• State the Law of Segregation. 2 alleles of the same gene will separate during meiosis and be placed into haploid gametes.

• Distinguish between linked and unlinked genes.

• Unlinked:  on different chromosomes and not attached

• Linked:  on same chromosomes and attached

• State the Law of Independent Assortment. Alleles on one gene will separate independently of the alleles on another gene. 

• line up independently of each other in  metaphase 1

• ex: seed color gene on chromosome 1 doesn’t affect seed texture gene on chromosome 2 

• exception: law of independent assortment does not apply to linked genes

• This means that the inheritance of one trait will not influence the inheritance of another trait, provided the genes are located on different chromosomes.