- REPRODUCTION, a basic characteristic of life, is defined as any process by which a cell or organism produces offspring.(Starr & Taggart, The Unity and Diversity of Life, 9th Edition, Glossary). This obviously requires CELL DIVISION.
- Cell division usually involves two steps. Nuclear division (KARYOKINESIS) and division of the cytoplasm (CYTOKINESIS).
- The events that take place between one cell division and the next are called the CELL CYCLE and are often identified as INTERPHASE.
- An organism's LIFE CYCLE includes the events that take place between its birth and its death.
- MITOSIS is the division of the nucleus of body (somatic) cells
into genetically identical daughter cells. MEIOSIS is division of the nucleus of cells in the reproductive organs and leads to the development of sex cells or GAMETES.
- The Cell Cycle and cellular reproduction are precisely regulated by a series of proteins called cyclins. The 2001 Nobel Prize in Physiology and Medicine was awarded to Dr. Paul Nurse and others who identified the chemical regulatory mechanisms of the cell cycle. This regulation includes checks on the division of the nucleus and the cytoplasm. Click here to read the press release and an explanation their work.
- It is crucial that each new reproducing cell get the proper amount and type of genetic information.
- Both the cell cycle and the life cycle of organisms involve growth periods.
- The nucleus of all eukaryotic cells is the central storehouse of information for the construction of the cell.
- CHROMOSOMES are molecules of DNA wrapped around histone proteins and contain the hereditary information within the sequence of nitrogen bases of the DNA molecule.
- Eukaryotes have variable numbers of chromosomes. The species number for humans is 46, for chimpanzees 48, for crayfish 300.
- Within an organism, all body cells have identical amounts and types of hereditary material. Thus all body cells such as skin cells, muscle cells, bone cells in the human have 46 chromosomes.
- The cell cycles in prokaryotes show similarities and
differences to that of eukaryotes.
- Since prokaryotes lack a nucleus and mitosis is a type of nuclear division, bacteria divide by a process termed BINARY FISSION.(Go to TOPICS, Microbiology, Dividing Bacteria)
- The circular DNA REPLICATES (makes an identical
copy) to form two side by side circles, each apparently with its own attachment to the cell membrane.
- The cell elongates and the daughter chromosomes are pulled apart.
- Cell contents are divided into the two cells.
- The cell cycle of eukaryotes includes a number of phases.
- INTERPHASE includes three growth phases. A cell undergoes growth and usually doubles in size during interphase.
- GAP 1 (G1) precedes the synthesis of new DNA. During this G1 phase the cell is accumulating materials such as water and the building blocks of organic molecules and building new parts. It is a young cell carrying on normal life functions. The length of this phase varies with the type of cell. Not all cells move directly into the S phase. Some cells move temporarily or permanently into what is known as the G0 phase. Certain cells, such as muscle and some nerve cells, leave the G1 phase and become specialized or differentiated and thus do not divide again. Other cells, such as the B lymphocytes which will eventually produce Antibodies, will return to the G1 phase when stimulated by an antigen. Upon their return they will continue in the cycle and eventually become plasma cells which will produce antibodies.
- During the S (synthesis) PHASE, an exact copy of the DNA is synthesized so that at the end of the phase there will actually be 46 pairs of chromosomes in the cell.
- During GAP 2 (G2), the cell is making final preparation including making proteins and preparing to divide. G1,G0,S, and G2 are collectively known as INTERPHASE.
- M (Mitosis) PHASE is the period of nuclear division.
- During mitosis, the two sets of chromosomes made
during the S phase are separated and two daughter nuclei form.
- During CYTOKINESIS, the daughter nuclei and cell
components are separated into daughter cells.
- Mitosis and Cytokinesis in Detail
- During M, chromatin become tightly wound up or condensed into visible structures called chromosomes. Each consists of two CHROMATIDS or arms and are held together at a central body, the CENTROMERE.
- The movement of the chromosomes and their separation
to opposite ends or poles of the cell is a continuous process but we like to slow the process down and think of a series of phases with obvious structural characteristics.
- In PROPHASE, chromosomes condense, the nucleolus disappears, and a MITOTIC SPINDLE forms. Later in this phase the nuclear membrane disappears and the
chromosomes attach to the spindle.
- In METAPHASE, the chromosomes become aligned (lined up) near the middle of the cell. This is the last checkpoint in mitosis. If spindle fibers have not attached to the centromeres the process stops here.
- In ANAPHASE, the centromeres divide and the spindle pulls the chromatids toward the poles. This is often the shortest stage.
- In TELOPHASE, the chromosomes arrive at opposite poles, the nuclear envelope reappears, and the spindle dissolves. In esssence, this is the opposite of prophase.
- Cytokinesis differs in plant and animal cells.
- In animal cells a contractile ring pinches the cell
forming a FURROW or CLEAVAGE FURROW which divides the cell in two. The movement is from the outside to the inside.
- In plant cells a central cell plate forms and the cell is divided into two. The movement is from the inside to the outside.
- There are a number of different ways in which cell division is controlled.
- Cells may exhibit CONTACT INHIBITION and stop
dividing when they touch other cells.
- Proteins called GROWTH FACTORS can enhance the growth
and division of particular cell types. These growth factors may be extrinsic or intrinsic.
- Chromosomes contain repeating sequences (TTAGGG) at their ends. Often called tails they are known as telomeres.
- Normal cells lose some of their telomeres each time they undergo division.
- Cancer cells replace their telomeres with the help of an enzyme known as telomerase.
- CANCER is the result of uncontrolled or unregulated cell multiplication.
- Two types of genes may be involved in regulation.
- Oncogenes are normally turned off.
- Tumor Suppressor genes are normally turned on. p53 is one of the most studied tumor suppressor genes.
- The life cycle is the process by which an immature individual matures and reproduces.
- ASEXUAL reproduction is the formation of an offspring
genetically identical to its parent.
- Asexual reproduction is common in plants.
- Certain animals reproduce asexually, by budding or regeneration.
- SEXUAL REPRODUCTION involves the fusion of GAMETES or
sex cells, produced by specialized cells in the sex organs.
- FERTILIZATION results in a single cell, the ZYGOTE
or fertilized egg. It restores the DIPLOID number of chromosomes.
- MEIOSIS is cell division of cells in the sex organs. The normal or species number of chromosomes in the body cells is the DIPLOID NUMBER. Meiosis reduces the diploid number of chromosomes by half to produce HAPLOID gametes.
- Genetic variation arises through genetic recombination
during meiosis.
- During meiosis, homologous chromosomes are randomly
distributed, a process of INDEPENDENT ASSORTMENT.
- CROSSING OVER is the exchange of parts between homologous chromosomes.
- Visit this site to see a number of differences between meiosis and mitosis.
- Meiosis reduces the number of chromosomes in daughter
cells by one-half. The diploid number is converted to the haploid or monoploid number. In Mitosis the chromosome number in the new daughter cells remains the same as the number of chromosomes present in the original parent cell. The diploid number is maintained.
- During mitosis, chromosomes are replicated exactly. During meiosis, chromosomes are replicated exactly but the sequences of alleles is changed during crossing over and thus the chromosomes have been modified after replication. This increases the variability of the gametes and thus of the species, adding more potential to be acted upon by natural selection.
- Mitosis occurs in body or somatic cell types throughout the
organism; meiosis occurs only in body cells of the reproductive organs.
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