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גנטיקה

מקצוע: ביולוגיה, כיתה: ט', מאת: Sally Mustang, פורסם: 15/05/2008
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סיכום באנגלית על תורשה וגנטיקה

Genetics

- The nucleus is built of chromosomes. Each chromosome is built of a line that is called DNA,
which holds together genes. Each gene decides the way a quality or characteristic will affect
the creature.
- Humans have 23 pairs of chromosomes. Those pairs are called Homological chromosomes.
Chromosomes do not pair up in reality, but when mapped, in a kariotype it is seen that one is
from the father and one is from the mother.
- Each cell nucleus in a human’s body (part reproduction cells) holds 46 chromosomes.
- Down syndrome is caused when on the 21st chromosome pair becomes a threesome (to give
the born 47 chromosomes in every cell).
- Mitosis is a process in which the cell multiplies and keeps the same number of chromosomes
(duplication). It consists of these stages:

- Interphase- The cell is engaged in metabolic activity and performing its prepare for mitosis
(the next four phases that lead up to and include nuclear division). Chromosomes are not
clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible.
The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both
of which are organizational sites for microtubules.

- Prophase- Chromatin in the nucleus begins to condense and becomes visible in the light
microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to
opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell
to form the mitotic spindle.

- Prometaphase- The nuclear membrane dissolves, marking the beginning of
prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules
attach at the kinetochores and the chromosomes begin moving.

- Metaphase- Spindle fibers align the chromosomes along the middle of the cell nucleus.
This line is referred to as the metaphase plate. This organization helps to ensure that in the
next phase, when the chromosomes are separated, each new nucleus will receive one
copy of each chromosome.

- Anaphase- the paired chromosomes separate at the kinetochores and move to opposite
sides of the cell. Motion results from a combination of kinetochore movement along the
spindle microtubules and through the physical interaction of polar microtubules.

- Telophase- Chromatids arrive at opposite poles of cell, and new membranes form around
the daughter nuclei. The chromosomes disperse and are no longer visible under the light
microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may
also begin during this stage.

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