OCR Biology A level 6.1.2 Patterns of inheritance summary notes

Cancer:
If cells have a mutation to genes involved in regulating mitosis
 Do not undergo apoptosis
 Keep on dividing by mitosis
 Tumour

Why?
 Reuse cell components
 No harmful enzymes released
 Weeds out ineffective/harmful cells
 Separation of limbs/digits during development




6.1.2 Patterns of Inheritance
Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the
associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are
expected, but not the subdivisions of prophase);

Meiosis
 Nuclear division where original number of chromosomes in the organism is halved
 Cells produced are haploid
 Means when two cells combined in sexual reproduction chromosome number is restored
 Produces non-identical cells

Meiosis I
Prophase
 Chromosomes supercoil and become visible
 Chromosomes consist of two SISTER chromatids (genetically identical)
 Chromosomes come together in their homologous pairs (each member has same
genes at the same loci) (one maternal, one paternal)
 Form bivalent

 Non-sister chromatids wrap around each other at points called chiasmata; "crossing
over"
o (they may break and swap sections of DNA - leading to alleles being switched
around)

 Nucleolus disappears
 Spindle forms
 Nuclear envelope breaks down
 Centrioles at opposite ends of cell


Metaphase
 Bivalents line up at equator
 Spindle fibres attached to bivalents at centromere

,  Bivalents line up randomly; independent assortment


Anaphase
 One homologous chromosome from each bivalent pulled to opposite poles
by spindle fibres
 Chiasmata separate
o (lengths of chromatid that have been crossed over remain with the
chromatid to which they have become newly attached)

Telophase
 Two new nuclear envelopes form
 New membrane form
 Cells split by cytokinesis
 Brief interphase; chromosomes uncoil

Meiosis II
Prophase
 Nuclear envelope breaks down
 Nucleolus disappears
 Chromosomes condense
 Spindles form at right angle to previous spindle axis
 Centrioles replicate and move to poles

Metaphase
 Chromosomes arrange themselves on equator of spindle
 Attached to spindle fibres at centromere
 Chromatids of each chromosome randomly assorted
 Different chromatids face each pole


Anaphase
 Centromeres divide
 Chromatids pulled to opposite poles by spindle fibres

Telophase
 Nuclear envelopes reform
 New cell membranes form
 Four haploid daughter cells (animals)
o (Plants - a tetrad of four haploid cell formed)


Note: If 3n, (e.g from 4n x 2n)
 Meiosis falls
 Ref homologous pairs
 Cannot form seeds

To get 3n..
 Mitosis
 Chromosomes replicated
 Failure of spindle

Key terms:

Allele:
 Version of a gene
 Difference in the DNA base sequence (that is expressed as a slightly different polypeptide)