Summary
OCR Biology A level 6.1.2 Patterns of inheritance summary notes
Summary notes for topic 6.1.2 Patterns of inheritance of OCR Biology A level Module 6. Detailed electronic notes with diagrams.
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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)
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