Topic 1 Flashcards
What is sexual reproduction
-Most animals and plants reproduce sexually
- sexual reproduction is the merging of genetic contribution form two cells of different individuals
- results in evolution
What is fertilisation?
- A human somatic cell contains 2 sets of chromosomes - diploid cell (2n)
- Gametes (egg or sperm) contain 1 set of chromosome- haploid cell (1n)
- Offspring inherit genetic material form both parents
- Fertlisation, also knows as syngamy, is the fusion of male gametes (sperm) with female gamete, producing a diploid zygote
describe diploid cells in detail
a diploid cell contains two version of each chromosome, the haploid maternal homologue from the female egg cell and the haploid paternal homologue from the male sperm cell
describe the sexual life cycle in humans
- in humans the zygote undergoes mitotic division, giving rise to all the cells in the adult body
- germ line cells are set aside early in development and undergoes meiosis to produce haploid gametes (egg cell or sperm cell)
- rest of body cell is called somatic cells
brief overview of meiotic division
- meiosis includes two rounds of division: meiosis 1 and meiosis 2
meiosis 1: prophase 1 then metaphase 1 then anaphase 1 then telophase 1
meiosis 2: prophase 2. metaphase 2, anaphase 2, telophase 2
the DNA replication occurs before meiosis 1
DNA replication does not occur between meiosis 1 and meiosis 2
first meiotic division is called as reduction division as the daughter cells (2 produced) consists of a homologue from each chromosome pair
second meiotic division separates the sister chromatids for each homologue
the 4 haploid daughter cells are directly develop into gametes after meiosis 2
prophase 1
- Chromosomes condense
- Microtubule spindle forms
- DNA replicated and the sister chromatids are held together at the centromere
- the homologous chromosomes pair along the entire length during synapsis
- crossing over occurs and forming chiasmata holding the homologous chromosomes together
metaphase 1
- pairs of homologous chromosomes align at the equator of the cell
- the chiasmata keeps the homologous pairs together
- microtubules from opposite ends of the cell attach to sister kinetochores of each homologue , producing tension
- a kinetochore microtubule of one pole attaches to the homologue of a chromosome
- the other kinetochore microtubule from the other cell pole attaches to the other homologue of the chromosome
anaphase 1
- kinetochore microtubules shorten causing one duplicated homologue to go to one pole of the cell and the other duplicated homologue goes to the other pole of the cell
- sister chromatids do separate unlike in anaphase of mitosis
- in mitosis the duplicated homologues line up in the metaphase plate and the sister chromatids are pulled apart to the opposite poles in the cell
telophase 1
- the separated homologues cluster at each pole of the cell
- nuclear envelope forms around the separated homologues forming 2 separate daughter nuclei
- cytokinesis may occur
- each daughter nuclei consists of half the number of chromosomes that the parent cell’s nucleus had
- each chromosome in a daughter cell has 2 sister chromatids but the sister chromatids are not identical to each other as crossing over has occurred
prophase 2
- after a brief interphase with no s phase, meiosis 2 occurs
- in prophase 2, a new spindle apparatus forms in each daughter cell
- nuclear envelope of each daughter cell breaks down
metaphase 2
- the chromosome consisting of 2 sister chromatids that are joined together at the centromere align at the metaphase plane of the cell
- the kinetochore microtubule from opposite poles of the cell attach to kinetochores of sister chromatids, as in mitosis
anaphase 2
- microtubules shorten
- sister chromatids are pulled to opposite poles of cell
telophase 2
- nuclear membrane forms around 4 different chromosome clusters
- after cytokinesis, resulting in 4 different haploid cells
- no two cells are alike due to the crossing over of two chromosomes in prophase 1 and random alignment of homologous pairs in metaphase 1
errors in meiosis
(nondisjunction)
- failure of chromosome to move to opposite poles during either meiotic division
- results in missing or extra chromosome called aneuploid gamete
- one of the most common causes spontaneous abortion in humans
