How do chromosomes divide

Autosomes are labeled for reference. Each chromosome pair consists of one chromosome inherited from the mother and one from the father. In addition to the 22 numbered autosomes, humans also have one pair of sex chromosomes called an allosome.

Instead of labeling these chromosome pairs with numbers, allosomes are labeled with letters such as XX and XY. Females have two copies of the X chromosome one inherited from the mother and one from the father.

Males have one copy of the X chromosome inherited from the mother and one copy of the Y chromosome inherited from the father. Arranged on the chromosomes are genes. Genes are made of DNA and contain the instructions for building proteins and are integral in making and maintaining the human body. Position of the gene on the arm cytogenetic bands. The position is dependent on the light and dark bands that appear on the chromosome when stained and is expressed as a two-digit number one digit represents region and one represents band.

Sometimes the digits are followed by a decimal point and one or more digits. These additional digits represent the distance from the centromere increasing numeric value indicates farther distance from centromere. Cen — close to the centromere Ter terminus — close to end of either the p or q arms Tel telomere — close to end of either the p or q arms. Example Gene: Anaplastic lymphoma kinase receptor Chromosomal location: 2p23 Location description: chromosome 2, p arm, position There are two types of cell division: mitosis and meiosis.

Meiosis is the type of cell division that creates egg and sperm cells. Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by certain genes. When mitosis is not regulated correctly, health problems such as cancer can result.

Anaphase I. The spindle tubules contract, pulling apart each member of each homologous pair of chromosomes to opposite poles in the cell. The arrangement of maternal and paternal chromosomes during metaphase I, and their subsequent segregation during anaphase I, is completely random.

This independent assortment of chromosomes ensures that spermatozoa and ova receive a good mix of maternal and paternal chromosomes. Telophase I.

The number of chromosomes at each pole of the cell has been reduced by half from 46 diploid number to 23 haploid number. A new nuclear membrane gradually forms around each haploid set of chromosomes, and cytokinesis leads to cleavage of the cytoplasm. This eventually produces two new haploid daughter cells. Each new haploid daughter cell now undergoes a second phase of cell division, meiosis II Fig 4.

The stages of meiosis II are, in most respects, identical to those of mitosis:. In men, spermatozoa are formed in the seminiferous tubules of the testes.

The germinal cells in the testes spermatogonia give rise to diploid primary spermatocytes, which then undergo meiosis resulting in four haploid spermatozoa.

Adult males produce huge numbers of spermatozoa at a rate of million per day. The number of ova produced by women during their reproductive years is significantly lower. The germinal cells of the ovaries oogonia give rise to diploid primary oocytes, which then undergo meiosis to form haploid ova oocytes.

Around two million ova are present at birth, but most of them progressively degenerate with age. This means that during her fertile years, a woman will only release, on average, around viable ova VanPutte et al, The key function of meiosis is to create gametes that have the haploid number of 23 chromosomes. With age, the separation of homologous chromosomes that occurs during meiosis becomes less efficient, which means that extra chromosomes may be carried over into the gametes.

This phenomenon is called nondisjunction. Nondisjunction commonly results in the ova of older women having an extra copy of chromosome Genes are the basic units of inheritance. The crossing over of chromosomes during meiosis and the independent assortment of chromosomes ensures that spermatozoa and ova have a random combination of genes inherited from the mother and the father.

This guarantees genetic diversity. Genes ultimately encode information for constructing the proteins that build our bodies and the enzymes that control our biochemistry. Part 3 will explore the translation of DNA sequences into proteins. Tagged with: Newly qualified nurses: systems of life. During this phase, the chromosomes inside the cell's nucleus condense and form tight structures.

In fact, the chromosomes become so dense that they appear as curvy, dark lines when viewed under a microscope Figure 1. Because each chromosome was duplicated during S phase, it now consists of two identical copies called sister chromatids that are attached at a common center point called the centromere. Figure 2: The mitotic spindle white begins to form outside the cell's nucleus.

Important changes also take place outside of the nucleus during prophase. In particular, two structures called centrosomes move to opposite sides of the cell during this phase and begin building the mitotic spindle. The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell Figure 2. After prophase is complete, the cell enters prometaphase.

During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid.

Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid Figure 3a.

Figure 3: a Metaphase and b Anaphase. In metaphase a , the microtubules of the spindle white have attached and the chromosomes have lined up on the metaphase plate. During anaphase b , the sister chromatids are pulled apart and move toward opposite poles of the cell. Figure Detail. After metaphase is complete, the cell enters anaphase.

During anaphase, the microtubules attached to the kinetochores contract, which pulls the sister chromatids apart and toward opposite poles of the cell Figure 3c. At this point, each chromatid is considered a separate chromosome. Figure 4: During telophase, two nuclear membranes form around the chromosomes, and the cytoplasm divides.

Finally, once anaphase is complete, the cell enters the last stage of the division process — telophase.