After a new nuclear membrane forms during telophase of mitosis or meiosis, the cytoplasm_________ divides, resulting in two cells. In plant cells, cytokinesis occurs when nucleus has divided and cell plate is formed.

prophase

During metaphase, the cell’s chromosomes align themselves in the middle of the cell through a type of cellular “tug of war.” The chromosomes, which have been replicated and remain joined at a central point called the centromere, are called sister chromatids.

The nucleus controls and regulates the activities of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary information. Nucleoli are small bodies often seen within the nucleus. The gel-like matrix in which the nuclear components are suspended is the nucleoplasm.

Within the nucleus, DNA molecules, the cell’s genetic machinery, are stored, repaired, transcribed and eventually replicated. Around the outside of the nucleus is an envelope consisting of two layers of membrane.

The nucleus (plural, nuclei) houses the cell’s genetic material, or DNA, and is also the site of synthesis for ribosomes, the cellular machines that assemble proteins. Inside the nucleus, chromatin (DNA wrapped around proteins, described further below) is stored in a gel-like substance called nucleoplasm.

The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles.

positive

orbital

The nucleus has an overall positive charge as it contains the protons. Every atom has no overall charge (neutral). This is because they contain equal numbers of positive protons and negative electrons.

The protons of an atom are all crammed together inside the nucleus. Each proton carries a positive charge, and like charges repel each other. However, forces in the nucleus counteract this repulsion and hold the nucleus together. (Physicists call these forces nuclear glue.

And again, the closer together they are, the stronger the repulsion. Now the nucleus of an atom is positively charged, while electrons are negatively charged. As a result, a nucleus will attract electrons. Furthermore, the protons are all positively charged, and so they all repel each other.

Size and Mass of the Nucleus Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense.

Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. It occurs concurrently with two types of nuclear division called mitosis and meiosis, which occur in animal cells.

Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells. During cell division, mitosis refers specifically to the separation of the duplicated genetic material carried in the nucleus.

When cells divide, they make new cells. A single cell divides to make two cells and these two cells then divide to make four cells, and so on. We call this process “cell division” and “cell reproduction,” because new cells are formed when old cells divide. The ability of cells to divide is unique for living organisms.

It happens throughout the entire lifespan of a living organism (human, animal or plant) but most rapidly during periods of growth. This means, in humans, the fastest rate of mitosis happens in the zygote, embryo and infant stage.

The two reasons why cell divides are: Growth. Replacing damaged or dead cells.

There are two ways cell division can happen in humans and most other animals, called mitosis and meiosis. When a cell divides by way of mitosis, it produces two clones of itself, each with the same number of chromosomes. When a cell divides by way of meiosis, it produces four cells, called gametes.

The cells of the skin and bone marrow are sites of active mitosis replacing skin cells and red blood cells that only have a limited 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.

Permanent cells are cells that are incapable of regeneration. These cells are considered to be terminally differentiated and non-proliferative in postnatal life. This includes neurons, heart cells, skeletal muscle cells and red blood cells.

In humans, meiosis is the process by which sperm cells and egg cells are produced. In the male, meiosis takes place after puberty. Diploid cells within the testes undergo meiosis to produce haploid sperm cells with 23 chromosomes. At puberty, meiosis resumes.

Females generally produce a few but large gametes, whereas males produce large numbers of much smaller gametes. Consequently, the limited number of gametes and the greater parental investment of the females result in an asymmetry in the degree of sexual selection between the two sexes.

Meiosis

• The process of meiosis happens in the male and female reproductive organs. As a cell divides to form gametes:
• Meiosis occurs in the testes of men and ovaries of women.
• Meiosis and mitosis differ because:

primordial germ cells

female ovary

Errors during meiosis can lead to mutations in gametes. Defective gametes that undergo fertilization may result in miscarriages or ultimately lead to genetic disorders. The most likely mistake to occur during meiosis is chromosomal non-disjunction, which results in the wrong number of chromosomes in a sex cell.

Mitosis occurs only in eukaryotic cells. For example, animal cells undergo an “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo a “closed” mitosis, where chromosomes divide within an intact cell nucleus.

Meiosis occurs in all animals and plants. The end result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly.

gamete cells

From Amy: Q1 = Cells undergoing mitosis just divide once because they are forming two new genetically identical cells where as in meiosis cells require two sets of divisions because they need to make the cell a haploid cell which only has half of the total number of chromosomes.

Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.

Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By the end of meiosis II, the diploid cell becomes four haploid cells.

Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells.