Interphase: Cell growth and DNA replication (duplication of DNA creates sister chromatid chromosome)
Meiosis I
- Prophase I: DNA supercoils and chromosomes condense, nuclear membrane dissolves, homologous pairs form bivalents, crossing over occurs
- Metaphase I: Spindle fibres from centrioles (at poles) attach to centromeres of bivalent, bivalents line up along the equator of the cell
- Anaphase I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
- Telophase I: Chromosomes decondense, nuclear membranes may reform, cell divide (cytokinesis) forming two haploid daughters cells
Interkinesis: An optional rest period between meiosis I and meiosis II, no DNA replication occurs in this stage
Meiosis II
- Prophase II: Chromosomes condense, nuclear membrane dissolves (if reformed), centrioles move to opposite poles (perpendicular to previous poles)
- Metaphase II: Spindle fibres form centrioles attach to centromeres of chromosomes, chromosomes line up along the equator of the cell
- Anaphase II: Spindle fibres contract and split the chromosome into sister chromatids, chromatids (now called chromosomes) move to opposite poles
- Telophase II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis), resulting in four haploid daughter cells
10.1.2 Outline the formation of chiasmata in the process of crossing over
Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis
The process of crossing over occurs as follows
- Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad)
- Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over)
- The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata
Chiasmata hold homologous chromosomes together as a bivalent until anaphase I
As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologous - these are called recombinant
10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I
During anaphase I, homologous chromosomes separate, such that each resultant daughter (and subsequent gametes) contains a chromosome of either maternal or paternal origin
The orientation of these homologous in metaphase I is random, such that there is an equal probability of the daughter cell having either the maternal or paternal chromosome
As humans have a haploid number of 23 chromosomes, this means that there is 223 potential gamete combinations (over 8 million combinations)
Crossing over in prophase I results in entirely new chromosome combinations, as recombination through gene exchange produces wholly original chromosomes containing both maternal and paternal DNA, resulting in near infinite genetic variability
Other sources of genetic variation include random fertilisations, DNA mutations, chromosome mutations and non-disjunction
10.1.4 State Mendal's law of independent assortment
Gregor Mendal was a 19th century Moravian monk who demonstrated that the inheritance of traits (i.e. genes) followed particular laws:
- Law of segregation: Each herediatary characteristics is controlled by two alleles, which segregate and pass into different reproductive cells (gametes)
- Law of independent assortment: The separation of alleles for one gene will occur independently of the separation of alleles for another gene
- According to the law of independent assortment, different allele combinations should always be equally impossible
- However this law only holds for genes that are on different chromosomes - the law of independent assortment does not apply to linked genes
10.1.5 Explain the relationship between Mendal's law of independent assortment and meiosis
The law of independent assortment relates to the random orientation of homologous chromosomes in metaphase I of meiosis
Because the orientation of a homologous pair is random, and does not affect the orientation of any other homologous pair, any one of a pair of alleles on a chromosome has an equal chance of being paired with, or separated from, any one of a pair of alleles on another chromosome
This means the inheritance of two different traits will occur independently of each other (provided the genes aren't linked)
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