12.1 How Do Cells Replicate?

• The basic steps in cellular replication are

  • copying the DNA (deoxyribonucleic acid)

  • separating the copies

  • dividing the cytoplasm to create two complete cells.

• Chromosome (“colored-body”) refers to these threadlike structures

• Research carried out since then has shown that a chromosome consists of a single long DNA double helix that is wrapped around proteins, called histones, in a highly organized manner

• DNA encodes the cell’ s hereditary information, or genetic material.

• A gene is a region of DNA in a chromosome that codes for a particular protein or ribonucleic acid (RNA).

• Each of the double-stranded DNA copies in a replicated chromosome is called a chromatid.

• Once mitosis begins, however, these connections are removed except for those at a specialized region of the chromosome called the centromere.

• Chromatid copies that remain attached at their centromere are referred to as sister chromatids.

• The most visually stimulating part of the show, called M (mitotic or meiotic) phase, occurs when cells are in the process of separating their chromosomes.

• The rest of the time, the cell is in interphase (“betweenphase”).

• S phase is part of the interphase. The process of copying the genetic material is separated, in time, from the partitioning of replicated chromosomes during M Phase.

• Howard and Pelc coined the term cell cycle to describe the orderly sequence of events that leads a eukaryotic cell through the duplication of its chromosomes to the time it divides.

• The gap between the end of M phase and the start of S phase is called G1 phase.

• The second gap, between the end of S phase and the start of M phase, is called G2 phase.

12.2 What Happens during M Phase?

• Eukaryotic chromosomes consist of DNA wrapped around globular histone proteins. This DNA-histone complex is called chromatin.

• Mitosis begins with the events of prophase which is when chromosomes condense into compact structures.

• The spindle apparatus is a structure that produces mechanical forces that:

  • move replicated chromosomes during early mitosis and

  • pull chromatids apart in late mitosis.

• Centrosome is a structure that contains a pair of centrioles

• Some of these microtubules extend from each spindle pole and overlap with one another-these are called polar microtubules.

• Removal of the envelope allows the cytoplasmic microtubules to attach to chromosomes at specialized structures called kinetochores.

• Once all the chromosomes have migrated to the middle of the spindle, the mitotic cell enters metaphase

• At this point, the chromosomes are lined up on an imaginary plane between the two spindle poles called the metaphase plate.

• The spindle poles are held in place partly because of

• Astral microtubules that extend from the MTOCs and interact with proteins on the plasma membrane.

• At the start of anaphase, the cohesins that hold sister chromatids together at 1e centromeres are cleaved by an enzyme

• During telophase, the nuclear envelope re-forms around each set of chromosomes, and the chromosomes begin to decondense

• In animals and many other eukaryotes, cytokinesis begins with the formation of a cleavage furrow

• Many bacteria divide using a process called binary fission.

12.3 Control of the Cell Cycle

• The substance that initiates M-phase in oocytes is now called the M-phase-promoting factor, or MPF.

• After further research, it was determined that MPF is made up of two distinct polypeptide subunits.

• MPF is a dimer consisting of a cyclin and a cyclin-dependent kinase

• MPF deactivation illustrates two key concepts about regulatory systems in cells:

  • Negative feedback occurs when a process is slowed or shut down by one of its products.

  • Destroying specific proteins is a common way to control cell processes.

• A cell cycle checkpoint is a critical point in the cell cycle that is regulated.

• In multicellular organisms, cells that keep dividing in this way may die or form a mass of cells called a tumor.

• The factors that are important in determining whether a cell passes the G1 checkpoint include:

  • Size

  • Availability of nutrients

  • Social signals

  • Damage to DNA

• Regulatory proteins such as p53 are called tumor suppressors.

• The four cell-cycle checkpoints have the same purpose: They prevent the division of cells that are damaged or that have other problems.

12.4 Cancer: Out-of-Control Cell Division

• Cancer is a general term for disease caused by cells that divide in an uncontrolled fashion, invade nearby tissues, and spread to other sites in the body.

• Cancerous cells cause disease because they use nutrients and space needed by normal cells and disrupt the function of normal tissues.

• Masses of noninvasive cells are noncancerous and form benign tumors.

• By spreading from the primary tumor site, cancer cells can establish secondary tumors elsewhere in the body. This process is called metastasis.

• Growth factors are polypeptides or small proteins that stimulate cell division.

• Serum is the liquid portion of blood that remains after blood cells and cell fragments have been removed.

• The Rb protein serves as a gatekeeper that enforces the G1 checkpoint.