Growth and repair, the cells in adults that don’t undergo mitosis are NEURONS and the sperms and the ova. Most of the time the cells are in interphase where DNA is uncoiled and is known as chromatin. Mitosis and meiosis are a continuous process
- Two daughter cells that are identical
- The first stage of MITOSIS is the duplication of the DNA genetic material, and this happens during INTERPHASE. The cells approximately double in size once DNA duplication is complete the cell will now move into PROPHASE
- PROPHASE is the initial stage where the DNA states to the coil and the chromosomes become visible. The chromosomes become shorter and thicker each half of the chromosomes is called a chromatid because each chromatid is genetically identical to the partner. Chromatids are called sister chromatids and are attached at a point called centromere. Which becomes visibly double later in MITOSIS. During prophase, a human cell has 92 chromatids which make 46 chromosomes which is DIPLOID NUMBER OF CHROMOSOMES FOUND IN HUMANS
- Other events that happen in prophase are that the nuclear membrane disintegrates and the organelles such as centrioles begin to migrate towards the opposite poles. One thing to notice when viewing the chromosomes under the microscope is that the size is different and the centromeres are in the different position
- METAPHASE – during this phase the nuclear membrane finally disappears, and the centrioles that begin to appear at opposite poles begin to produce spindles, which are composed of microtubules. These spindles push and pull into approx. The middle of the spindles. This is known as the METAPHASE PLATE. Signifies that the chromosomes are lined in the equator of the cells.
- ANAPHASE – begins when the centromere visibly double and begin to separate. The moment the centromere split the chromosomes are considered to have divided, and each half that is previously known as chromatid is now known as a chromosome. When the centromere split, the halves are carried towards opposite centrioles by the spindles, which shorten this process is known DISJUNCTION.
- TELOPHASE – a nuclear envelope begins to appear around the individual chromosomes, the spindles break down, and cytokinesis happens, and there is now two genetically daughter cells and the chromosomes in each of the daughter cells uncoil assuming their interphase appearance.
- Happens in the spermatocytes in males and oocytes in females it begins with an INTERPHASE which is called INTERPHASE 1, AND IT IS DURING THIS PHASE THAT DNA REPLICATION HAPPENS
- PROPHASE 1 – marked by the appearance of a long thread like chromosomes this is the most important of meiosis because so many events happen. Chromosomes become shorter and thicker. The carrying some genetic loci come to lie side by side and gene for gene. The pair first near the tips and pair in the process something like a zipper being closed. The process is called SYNAPSIS. AT THIS POINT THERE ARE 23 STRUCTURES VISIBLE EACH OF THEM MADE UP OF 2 HOMOLOGOUS PAIRED ALONG THERE LENGTH
- When synapsis is finished the chromosomes become visibly doubled each chromosome consists of a pair of sister chromatids attached at a single centromere. So the synapse homologue consists of two centromeres side by side each centromeres attaching to 2 sister chromatids this structure is called a tetrad or bivalent. Chromatids that are attached to different centromere within a tetrad are called NON-SISTER CHROMATIDS
- You can see that each tetrad is made of 4 sister chromatids are replicas of different chromosomes and may be genetically different
- There are points within the tetrad where chromosomes come into contact, for example, non-sister these points are called CHIASMA and are points where crossing happens. Between non-sister chromatids which is the exchange of the parts of the chromatids, the end is the recombination of the genetic material. Also happening in prophase 1 is the movement of the centrioles to the opposite poles and the formation of the spindles the tetrad move to the periphery or edge of the nucleus and the nuclear envelope breaks down which is the end of prophase 1
- METAPHASE 1- the spindle is complete, and the bivalent are moved so that they lie in the equatorial plane lined up so that the 2 centromeres lie on opposite sides of the plane point to the poles of the spindle
- ANAPHASE 1 – happens when centromere of each bivalent separate being pulled in opposite direction by the spindles each taking their pair of chromatids with them. As the chromatids move towards the poles, they lose their attraction for each other and become floppy but still stay attached to some centromere. The result of this disjunction is the halving of the chromosomal number in what is called a reductional division which results in HAPLOID NUMBER OF CHROMOSOMES that is they will be 23 non-homologous chromosomes at each pole.
- TELOPHASE – chromosomes uncoil slightly, and cytokinesis happens. In human roughly an equal amount of the cytoplasm are divided among the 2 daughter cells, and each of the two daughter cells undergoes the following stages of meiosis In females, cytoplasm division is very unequal one cells receiving most of it, and this cell gives rise to the oocyte the other cell is very small and is called a polar body
- After the cytoplasmic division, the cells undergo another division called INTERPHASE 2 where there is a further uncoiling of the DNA but no DNA replication
- PROPHASE 2 – the chromatids coil and in metaphase 2 the chromosomes line up in the middle plane
- ANAPHASE 2 the centromeres split, and each chromatid becomes chromosomes with its own centromere moving towards the opposite poles of the cells
- In TELOPHASE – the chromosomes uncoil nuclear envelope reform, and the cytoplasm divides, and this ends meiosis
In TELEPHASE 1 – in male’s cytoplasmic division results in two daughter cells with roughly equal size? In females, the division is very unequal so that an oocyte and another polar body is formed. The first polar body that is formed at the end of telophase 1 will often also undergo second meiotic division so that at the end of miosis there may be 3 polar bodies but only one oocyte
- At the end of meiosis, the primary spermatocytes will give rise to 4 spermatozoa each with 23 chromosomes
- And in females, the original cell will give rise only one oocyte and up to three polar bodies the oocyte containing 22 autosomes an X chromosome
- Following TELOPHASE 2 the nuclei undergo further differentiation necessary to form a mature oocyte or sperm
- EARLY in fetal life, primitive germ cells called OOGONIA proliferate by mitotic division
- All OOGONIA enlarging forming primary oocytes before birth. 2 million of this oocyte in the ovaries of the newly born female
- As the primary oocyte form, they are surrounded by a layer of epithelial follicular cells called PRIMORDIAL FOLLICLE. During childhood, many of these primordial follicles will regress so that by puberty only 40,000 is left
- As the primary follicles enlarge at puberty, the flattened follicular cells become columnar forming a primary follicle
- The primary cells are then surrounded by an intracellular material called the ZONA PELLUCIDA . when primary follicles have more than one layer of cells it is called a growing follicle the cells around the pellucida become a structure called the CORONA RADIATA
- Primary oocyte begin its first meiotic division before birth, but they don’t finish prophase 1 until puberty
- As the follicles mature after puberty the primary oocyte increases in size until shortly before ovulation the secondary oocyte begin the second meiotic division but only as METAPHASE 2
- IS THE SECONDARY OOCYTE IS PENETRATED BY THE SPERM THE SECOND MEIOTIC DIVISION IS COMPLETE. The second polar body is degenerating when this happens the mature oocyte is commonly referred to them as mature ovum.
- About 400 secondary oocytes are released during a female lifetime
KARYOTYPES AN NON DISJUNCTION
- The result of non-disjunction in either ANAPHASE 1 OR 2
- The first syndrome that is associated with this is trisomy 21 known as down syndrome individual with this syndrome the karyotype 47+G
- TURNER SYNDROME – with the karyotypes 45X or 45XO, short female an sterile also congenital abnormalities such as the webbing of the skin of the neck
- KLINEFELTER – 47XXY male, sterile, tall, small testis and maybe mentally retarded
Bovellan, M., Romeo, Y., Biro, M., Boden, A., Chugh, P., Yonis, A., … & Jégou, A. (2014). Cellular control of cortical actin nucleation. Current Biology, 24(14), 1628-1635.
Newman, T., McGehee, J., Cahoon, C., Elnatan, D., Bui, B., Chu, D., & Burgess, S. (2016). Chromosome Organization in Meiosis. Molecular Biology of the Cell, 27, Art-No.