The answer is pronuclei. A pronuclei is the state of the
nucleus of either the sperm or the ovum after the sperm has entered the ovum
but the two nuclei have not yet fused. When the sperm enters the ovum, the membrane
of the sperm pronuclei dissolves hence leaving no barrier for the two genetic
materials to combine.
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The purpose of mitosis is to create two new perfectly identical cells when either there is a need to replace old or damaged cells and to reproduce asexually by making new cells. Some organisms use mitosis to replace body parts. For example starfish replace lost arms by mitosis. Some organisms such as the hydra use mitosis to produce genetically identical offspring.
There are four stages of mitosis: prophase, metaphase, anaphase and telophase.
1) Prophase: chromatin into chromosomes, the nuclear envelope break down, chromosomes attach to spindle fibres by their centromeres
2) Metaphase: chromosomes line up along the metaphase plate (centre of the cell)
3) Anaphase: sister chromatids are pulled to opposite poles of the cell
4) Telophase: nuclear envelope reforms, chromosomes unfold into chromatin, cytokinesis can begin
● The order of the stages of mitosis can be remembered using the mnemonic PMAT.
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Vacuoles are essentially sacs surrounded by a membrane. They are used by cells as temporary storage sites. They often store food, enzymes, and other materials needed by the cell, and some vacuoles store waste products.
The medulla, pons, and the cerebellum are three main parts of the hindbrain.
Brain is the centre of the nervous system. It is enclosed in the skull cranium. The primary regions of the brain are the forebrain, midbrain and the hindbrain. The forebrain consists of the cerebrum and diencephalon. The midbrain is a small tubular region between the forebrain and the hindbrain. The hindbrain is made up of the medulla oblongata, pons and cerebellum.
Cerebellum is the second largest portion of the brain which helps in maintaining the body's equillibrium, coordinates muscular movements and body posture. Pons is the structure located in the brain stem which relays impulses from medulla to cerebellum and vice versa. Medulla oblongata is the lowest part of the brain stem controlling involuntary reflexes and has the respiratory and cardiac centres in it controlling respiration and heart beats respectively.
