8.1 - 8.3 Flashcards
(22 cards)
Note 1 —-»
The ability to transmit INFORMATION is one of the unifying themes that encompasses all levels of biological study. Such information flow is absolutely necessary for reproduction. Only people can make more people and only maple trees can make more maple trees because each species carries and transmits its own specific genetic information at the cellular level. When a cell undergoes reproduction or cell division, the two “daughter” cells that result are genetically identical to each other and to the original “parent” cell. (Biologists traditionally use the word daughter in this context; it does not imply gender.) Before the parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the cell’s genetic information in the form of DNA. Then, during cell division, one set of chromosomes is distributed to each daughter cell. As a rule, the daughter cells receive identical sets of chromosomes from the lone, original parent cell. Each offspring cell will thus be genetically identical to the other and to the original parent cell.
Cell Division
The reproduction of a cell through duplication of the genome and division of cytoplasm.
Chromosome
A gene-carrying structure found in the nucleus of a eukaryotic cell and most visible during mitosis and meiosis.
Note 2 —-»
Sometimes, cell division results in the reproduction of a whole organism. Many single-celled organisms, such as prokaryotes or the eukaryotic yeast cell reproduce by dividing in half, and the offspring are genetic replicas. This is an example of asexual reproduction, the creation of genetically identical offspring by a single parent, without the participation of sperm and egg. An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals. Many multicellular organisms can reproduce asexually to produce clones. For example, some sea star species and many house plants have the ability to grow new individuals from fragmented pieces. In asexual reproduction, there is one simple principle of inheritance: The lone parent and each of its offspring have identical genes.
Sexual Reproduction
The creation of genetically unique offspring by the fusion of two haploid sex cells, forming a diploid zygote.
Note 3 —-»
The production of gametes involves a particular type of cell division that occurs only in reproductive organs (testes and ovaries in humans). A gamete has only half as many chromosomes as the parent cell that gave rise to it, and these chromosomes contain unique combinations of genes. In contrast to a clone, offspring produced by sexual reproduction are not identical to their parents or to each other (with the exception of identical twins), although they generally resemble their parents more closely than they resemble unrelated individuals of the same species. They are variations on a common theme of family resemblance, not exact replicas. Each offspring inherits a unique combination of genes from its two parents and this one-and-only set of genes programs a unique combination of traits. As a result, sexual reproduction can produce great variation among offspring.
Note 4 —-»
In addition to the production of gametes, cell division plays other important roles in multicellular organisms. Cell division enables sexually reproducing organisms to develop from a single cell—the fertilized egg, or zygote—into an adult organism. All of the trillions of cells in your body arose via repeated cell divisions that began in your mother’s body with a single fertilized egg cell. After an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or from accidents. Within your body, millions of cells must divide every second to replace damaged or lost cells. For example, dividing cells within your epidermis continuously replace dead cells that slough off the surface of your skin.
Note 5 —-»
The type of cell division responsible for the growth and maintenance of multicellular organisms and for asexual reproduction involves a process called mitosis. The production of egg and sperm cells involves a different type of cell division called meiosis. In the remainder of this chapter, you will learn the details of both mitosis and meiosis.
What function does cell division play in an amoeba (a single-celled protist)? What functions does it play in your body?
Reproduction in amoeba; development, growth, and repair in our body
Note 6 —-»
Prokaryotes (single-celled bacteria and archaea) reproduce by a type of cell division called binary fission, a term that means “dividing in half.” In typical prokaryotes, most genes are carried on one circular DNA molecule that, with associated proteins, constitutes the organism’s single chromosome. Although prokaryotic chromosomes are generally much shorter than those of eukaryotes, duplicating them in an orderly fashion and distributing the copies equally to two daughter cells are still formidable tasks. Consider, for example, that when stretched out, the chromosome of the bacterium Escherichia coli (E. coli) is about 500 times longer than the cell itself. It is no small feat to accurately replicate this molecule when it is coiled and packed inside the cell.
Binary Fission
A means of asexual reproduction in which a parent organism, often a single cell, divides into two genetically identical individuals of about equal size.
Note 7 —-»
- As the chromosome is duplicating, one copy moves toward the opposite end of the cell.
- Meanwhile, the cell elongates.
- When chromosome duplication is complete and the cell has reached about twice its initial size, the plasma membrane pinches inward and more cell wall is made, which eventually divides the parent cell into two daughter cells
Why is binary fission classified as asexual reproduction?
Because the genetically identical offspring inherit their DNA from a single parent
Note 8 —-»
Eukaryotic cells, in general, are more complex and much larger than prokaryotic cells. In addition, eukaryotic cells usually have many more genes, the units of information that specify an organism’s inherited traits. Human cells, for example, carry just under 21,000 genes, versus about 3,000 for a typical bacterium. Almost all the genes in the cells of humans, and in all other eukaryotes, are found in the cell nucleus, grouped into multiple chromosomes. (The exceptions include genes on the small DNA molecules within mitochondria and, in plants, within chloroplasts.) Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus. For example, human body cells have 46 chromosomes, while the body cells of a dog have 78 and those of a hedgehog have 90.
Note 9 —-»
Each eukaryotic chromosome consists of one long DNA molecule—bearing hundreds or thousands of genes—and a number of protein molecules, which are attached to the DNA. The proteins help maintain the chromosome’s structure and control the activity of its genes. Together, the entire complex—consisting of roughly equal amounts of DNA and protein—is called chromatin.
Chromatin
The complex of DNA and protein that makes up eukaryotic chromosomes.
Note 10 —-»
Most of the time, chromatin exists as a diffuse mass of long, thin fibers that, if stretched out, would be far too long to fit in a cell’s nucleus. In fact, the total length of DNA in just one of your cells exceeds your height! Chromatin in this state is too thin to be seen using a light microscope.
As a cell prepares to divide, its chromatin coils up, forming tight, distinct chromosomes that are visible under a light microscope. Why is it necessary for a cell’s chromosomes to be compacted in this way? Imagine that you have to move. Your belongings are spread throughout your home, but as you prepare to move, you gather them up and pack them into small containers to make them more easily sorted and transported. Similarly, before a cell can undergo division, it must compact all its DNA into manageable packages.
Note 11 —-»
The chromosomes of a eukaryotic cell are duplicated before they condense and the cell divides. The DNA molecule of each chromosome is replicated, and new protein molecules attach as needed to maintain the chromosome’s structure and regulate its genes. Each chromosome now consists of two copies called sister chromatids, joined copies of the original chromosome. The two sister chromatids are attached together along their lengths by proteins, most closely at a region called the centromere (visible as a narrow “waist” near the center of each chromosome shown in the figure).
Sister Chromatids
One of the two identical parts of a duplicated chromosome in a eukaryotic cell.
Centromere
The region of a duplicated chromosome where two sister chromatids are joined and where spindle microtubules attach during mitosis and meiosis.
Note 12 —-»
When the cell divides, the sister chromatids of a duplicated chromosome separate from each other. Once separated from its sister, each chromatid is considered an individual chromosome, and it is identical to the cell’s original chromosome. During cell division, one of the newly separated chromosomes goes to one daughter cell and the other goes to the other daughter cell. In this way, each daughter cell receives a complete and identical set of chromosomes. In humans, for example, a typical dividing cell has 46 duplicated chromosomes (and thus 92 chromatids), and each of the two daughter cells that results from it has 46 single chromosomes.
When does a chromosome consist of two identical chromatids?
When the cell is preparing to divide and has duplicated its chromosomes but before the duplicates actually separate.
