MB 12.1

How does a chromosome get duplicated into two daughter cells?

1. One of the multiple chromosomes in a eukaryotic cell is represented here, not yet duplicated. Normally it would be a long, thin chromatin fiber containing one DNA molecule and associated proteins. The chromosome duplicates (which includes DNA replication) and condenses. 2. Once duplicated, a chromosome consists of two sister chromatids connected along their entire lengths by sister chromatid cohesion. Each chromatid contains a copy of the DNA molecule. The sister chromatids will then separate into two chromosomes. 3. Molecular and mechanical processes separate the sister chromatids into two chromosomes and distribute them to to daughter cells.

How long is the DNA in a typical human cell?

A cell's endowment of DNA, its genetic information, is called its genome. Although a prokaryotic genome is often a single DNA molecule, eukaryotic genomes usually consist of a number of DNA molecules. The overall length of DNA in a eukaryotic cell is enormous. A typical human cell, for example, has about 2 meters of DNA - a length about 250,000 times greater than the cell's diameter. Yet before the cell can divide to form genetically identical cells, all of this DNA must be copied or replicated, and then the two copies must be separated so that each daughter cell ends up with a complete genome.

What roles does cell division play in life?

Cell division plays several important roles in life. The division of one prokaryotic cell reproduces an entire organism. The same is true of a unicellular eukaryote. Cell division also enables multicellular eukaryotes to develop from a single cell, like the fertilized egg that gives rise to a two-celled embryo. And after such an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents. For example, dividing cells in your bone marrow continuously make new blood cells.

What are sister chromatids?

Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. The two chromatids, each containing an identical DNA molecule, are initially attached along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion.

What is a centromere?

Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA sequences and gives the condensed, duplicated chromosome a narrow "waist". The part of a chromatid on either side of the centromere is referred to as an arm of the chromatid. (An uncondensed, unduplicated chromosome has a single centromere and two arms.)

What are somatic cells?

Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. For example, the nuclei of human somatic cells (all body cells except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent.

How many chromatids are in a duplicated chromosome?

I believe two.

How many chromosomes do the gametes in humans carry?

In contrast, you produce gametes-eggs or sperm- by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes, half as many chromosomes as the parent cell.

What is mitosis?

Later in the cell division process, the two sister chromatids of each duplicated chromosome seperate and move into two new nuclei, one forming at each end of the cell. Once the sister chromatids separate, they are no longer called sister chromatids but are considered individual chromosomes. Thus each new nucleus receives a collection of chromosomes identical to that of the parent cell. Mitosis, is the division of the genetic material in the nucleus, which is usually followed immediately by cytokinesis, the division of the cytoplasm. Once cell has become two, each the genetic equivalent of the parent cell.

Where does meiosis occur in humans?

Meiosis in humans occurs only in the gonads (ovaries or testes). In each generation, meiosis reduces the chromosome number from 46 (two sets of chromosomes) to 23 (one set). Fertilization fuses two gametes together and returns the chromosome number to 46, and mitosis conserves that number in every somatic cell nucleus of the new individual.

How many somatic cells does mitosis and cytokinesis produce by the time you are an adult?

Mitosis and cytokinesis produced the 200 trillion somatic cells that now make up your body and the same processes continue to generate new cells to replace dead and damaged ones.

What are gametes?

Reproductive cells, or gametes - sperm and eggs - have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans. The number of chromosomes in somatic cells varies widely among species: 18 in cabbage plants, 48 in chimpanzees, 56 in elephants, 90 in hedgehogs, and 148 in one species of alga.

What is one of the characteristics that best distinguishes living things from nonliving matter?

The ability of organisms to produce more of their own kind is the one characteristic that best distinguishes living things from nonliving matter. This unique capacity to procreate, like all biological functions, has a cellular basis. Rudolf Virchow, a German physician, put it this way in 1855: "Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and the plant from a plant". He summarized this concept with the Latin axiom "Omnis cellula e cellula," meaning "Every cell from a cell." The continuity of life is based on the reproduction of cells, or cell division.

What will you learn in chapter 12?

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell into its own division into two daughter cells. (Our use of the words daughter or sister in relation to cells is not meant to imply gender.) Passing identical genetic material to cellular offspring is a crucial function of cell division. In this chapter, you will learn how this process occurs. After studying the cellular mechanics of cell division in eukaryotes and bacteria, you will learn about the molecular control system that regulates progress through the eukaryotic cell cycle and what happens when the control system malfunctions. Because a breakdown in cell cycle control plays a major role in cancer development, this aspect of cell biology is an active area of research.

A chicken has 78 chromosomes in its somatic cells. How many chromosomes did the chicken inherit from each parent? How many chromosomes are in each of the chicken's gametes? How many chromosomes will be in each somatic cell of the chicken's offspring?

The chicken inherited 39 chromosomes from each parent. There will be 39 chromosomes in each of the chicken's gametes. There will be 78 chromosomes in each somatic cell of the chicken's offspring. This will all happen if the chicken does contract any disease or have any mutations, that would take away or add chromosomes.

What are chromosomes?

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into structures called chromosomes, so named because they take up certain dyes used in microscopy (from the Greek "chroma", color, and "soma", body. Each eukaryotic chromosome consist of one very long, linear DNA molecule associated with many proteins. The DNA molecule carries several hundred to a few thousand genes, the units of information that specify an organism's inherited traits. The associated proteins maintain the structure of the chromosome and help control the activity of the genes.

What happens to DNA when a cell experiences cell division?

The reproduction of an assembly as complex as a cell cannot occur by a mere pinching in half; a cell is not like a soap bubble that simply enlarges and splits in two. In both prokaryotes and eukaryotes, most cell division involves the distribution of identical genetic material - DNA - to two daughter cells. (The exception is meiosis, the special type of eukaryotic cell division that can produce sperm and eggs.) What is most remarkable about cell division is the fidelity (faithfulness) with which the DNA is passed along from one generation of cells to the next. A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

What is chromatin?

Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin. As you will see soon, the chromatin of a chromosome varies in its degree of condensation during the process of cell division.

Are chromosomes long and thin or densely coiled after DNA replication?

When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber. After DNA replication, however, the chromosome condense as part of cell division: Each chromatin fiber becomes densely coiled, and folded, making the chromosomes much shorter and so thick that we can see them with a light microscope.

How does a zygote get formed?

You inherited 46 chromosomes, one set of 23 from each parent. They were combined in the nucleus of a single cell when a sperm from your father united with an egg from your mother, forming a fertilized egg, or zygote.