f215 module 1

 Chapter 2

a. Describe the behaviour of chromosomes during meiosis and the associated behaviour of the nuclear envelope, cell membrane and centrioles.

Meiosis I:

Prophase I

Chromatin condenses and undergoes supercoiling so shorten and thicken.

Chromosomes come together in their homologous pairs to form a bivalent. Each pair has the same genes at the same loci and a maternal and paternal chromosome.

Non-sister chromatids wrap around each other at attach points known as chismata.

They may swap sections of chromatids known as Crossing over.

The nucleolus disappears.

The nuclear envelope disintegrates.

Spindle forms which is made up of protein microtubules.

Metaphase I

Bivalents line up across the equator of the spindle attached to spindle fibres. The chismata are still present.

Random assortment of the bivalents takes place with each member of a homologous pair facing opposite poles.

Prophase I

The homologous chromosomes are pulled apart by the spindle fibres to opposite poles.

The chismata separate and the lengths of chromatid that have been crossed over stay attached to the newly attached chromatid. 

Telophase I

In animal cells, two new nuclear envelopes form around each set of chromosomes at the poles. Cytokinesis takes place to divide the cell. The chromosomes uncoil. 

In plant cells, the cell goes from anaphase I to meiosis II

Meiosis II:

Prophase II

Nuclear envelope is broken down.

Nucleolus disapperars.

Chromosomes condense.

Spindles form.

Metaphase II

Chromosomes arrange themselves on the equator of the spindle and attach to the spindle fibres.

Random arrangement of the chromatids of the chromosome takes place.

Anaphase II

Centromeres divide.

Chromatids pulled to opposite poles by the spindle fibres.

Chromatids randomly segregate.

Telophase II

Nuclear envelope reforms.

In animals, the two cells divide into four haploid cells.

In plants, a tetrad of four haploid cells is formed.

b. Explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive.

Allele is an alternative version of a gene

Locus is the Specific position on a chromosome which is occupied by a particular gene.

Phenotype is the characteristics that are expressed in the organism and that can be observed. It is determined by the genotype and the environment around it during the development.

Genotype is The genetic makeup of an organism and is described by the alleles that the organism contains. This is not able to be observed.

Dominant is An allele which is always expressed in the phenotype.

Recessive is An allele which is only expressed in the phenotype if there is not a dominant allele present.

Codominant is when Two alleles of the same gene that are always expressed in the phenotype of a heterozygote.

c. Explain the terms linkage and crossing over

• Linkage is When two or more genes are located on the same chromosome. The linked alleles are normally inherited together as they do not segregate independently in meiosis. Sex linkage is a type of linkage where the gene that code for something is only found on one of the sex chromosomes (either X or Y).
• Crossing over is…