Intro to Cytogenetics Flashcards
Somatic division
- mitosis
- results in duplication of the contents of the cell including the DNA followed by cytokinesis or separation of the materials into two new daughter cells
- with respect to the DNA content the original cell is dipoid (2N)
- after the DNA replicates the DNA content becomes 4N but the original constitution (2N) is reestablished following division
- five steps: interphase, prophase, metaphase, anaphase, and telophase
Interphase
- the DNA replicates so the 2N cell becomes 4N
- the newly relpicated chromatids are held together and appear as one structure, so there is no increase in the number of chromosomes although the DNA content has doubled
Metaphase
-the chromosomes shorten and thicken and can be visualized by light microscopy
Anaphase
-centromeres divide and the chromosomes separate
Cytokinesis
- the final step in cell division occurs in telophase
- the net result is 2 daughter cells that should be copies of the original parent cell
- each daughter cell is 2N
Meiosis
- germ cells undergo a different type of cell division
- occurs in gonads
- has two different events recombination of linked alleles leads to the reassortment of the genes
- the hallmark of meiosis is reduction division which is a reduction by half of the original number of chromosomes
- this is accomplished by two consecutive cell divisions, but only a single DNA duplication
- thus a cell begins as 2N but the final product is N
Recombination
- when portions of two homologues break and exchange places
- this has been visualized and the event is known as crossing-over. If you look at the allelic distribution of genes within the families you will see a patchwork of different alleles attesting to the recombination in meiosis
Crossover
-an exchange between homologous chromosomes resulting in a reassortment of the genes/allelespresent on each chromosome
Steps of meiosis
1) A dipoid cell with 2C DNA replicates giving rise to a 2N cell with a 4C DNA content
2) At metaphase I, the homologous chromosome pair and in the first division (anaphase I) the centromeres remain together and the homologs separate such that one chromosome of each pair is pulled to the opposite pole of the cell. This is reduction division the crucial step in meiosis
3) The 2N cell has split into 2 daughter cells- both with half the total number of chromosomes so they are haploid or N. However, each chromosome in the daughter cells is a duplex (each chromosome has 2 chromatids, so the total DNA content of the cell is 2C
4) A second division must occur. This division is similar to a mitotic division without DNA replication
5) In each daughter cell, the chromosomes line up, the centromeres divide, and the chromosomes are pulled to opposite poles
6) Each daughter cell receives one chromosome, but there are NO pairs of chromosomes. Each chromosome is unique. These daughter cells have half the original total of chromosomes and half the original DNA content
Meiotic Nondisjunction
- failure of chromosome or chromatids to disjoin propertly can occur
- in meisos nondisjunction can occur at either the first or second division and the net result is different
Disomy
- the presence of 2 chromosomes
- isodomy: 2 chromosoms from the same source -> duplication of 1 chromosome
- heterodisomy: 2 different chromosome
Meiotic Nondisjunction I
- happens in Anaphase I
- if nondisjunction at meiosis I occurs, A1 and A2 may NOT separate. This will give rise to a cell with both A chromosomes and the other with No A chromosomes
- assuming the second division occurs correctly, the net result will be 4 cells with a chromosomal imbalance
- two different copies of A = heterodisomic for A
- missing A = nullosomic for A
Meiotic Nondisjunction II
- non disjunction happens in meiosis II
- there is just two cells with chromosomal imbalacne
- one cell disomic and one nullosomic
Gametogenesis
=how gametes are formed
- spermatogenesis- occurs in males and produce sperm
- an original germ cell replicates via mitosis to generate a large population of spermatogonia which are 2N. At some point one of these cells will enter meiotic division. The DNA replicates giving a primary spermatocyte. This cell divides and following reduction division becomes a secondary spermatocyte. The second meiotic division results in haploid spermatids which will mature into sperm. Each primary spermatocyte gives rise to 4 spermatids. This is a classic meiotic division
- oogenesis occurs in females and produces the eggs
Oogenesis
- oogonia replicate via mitosis. One of these cells will enter meiosos and become a primary oocyte
- the first meiotic division results in two unequal daughter cells
- both daughters receive equal halves of the DNA but one daughter receives the majority of the cytoplasm. This is the secondary oocyte. The daughter with little cytoplasm is known as the first polar body. This cell usually degrades, though it can go through a second division giving rise to secondary polar bodies
- 2nd oocyte goes through second meiotic division two daughter cells one has more cytoplasm which is the egg cell
- only one functional gamete
Timing of oogenesis
- begins in fetus
- by 3rd month gestation the primary oocytes are present
- these cells reach dictyotene (prophase 1) by birth and remain so until ovulation up to 50 years later
- at ovulation, the oocyte completes meiosis I becoming a secondary oocyte
- meiosis will only be completed if fertilization occurs
- penetration of the sperm head stimulates the final division and release of the second polar body
- it is then possible for the male and female pronuclei to fusion creating a zygote
