Science at a Distance
This Bio-Module requires the use of the text book " Exploring Life" by Professor John Blamire.
a check up
Use this department to check up on the accuracy of your lecture notes. Make sure that you have written down the following definitions, explanations and important concepts in your notes.
Biological Information - Part Three
Mechanisms of Inheritance
Biological information can be passed on from one generation to the next. Each gene, each trait, is faithfully copied and then the copy transferred to the new cell or organism as it reproduces and duplicates itself.
- reproduction is a basic and fundamental process common to all forms of life on earth.
- in this process organisms and cells produce new copies of themselves by following the instructions and blueprints they all carry.
- during reproduction these instructions and blueprints, in the form of biological information, are copied and then passed on from one generation to the next. This is inheritance.
- cellular life depends on the ability of cells to grow, copy their biological information and then divide into two new cells while ensuring that each new cell receives a complete copy of all the information it needs. This is cell division.
- multicellular organisms reproduce by the production of specialized reproductive cells which fuse and combine to produce the start of a new individual. During this process biological information is also copied and inherited.
Cell Cycle and Life Cycle
All eukaryotic cells pass through a series of well defined stages as they grow, replicate their DNA, prepare, divide up the copies of the DNA molecules and divide. This is the cell cycle. Multicellular organisms also pass through recognized stages of growth, DNA replication, production of reproductive cells and the generation of new individuals who inherit DNA molecules. This is a life cycle
- almost all eukaryotic cell cycles consist of four recognizable phases; G1 (a period of growth), S (a period of DNA replication and synthesis), G2 (a second period of growth and preparation) and M (a period in which the cells divide up their DNA and two new cells separate).
- cell division results in two new cells each of which are genetically identical to each other and to the original parental cell.
- some multicellular organisms also reproduce by producing genetically identical clones of themselves.
- any form of reproduction that results in new individuals that are genetically identical is called asexual reproduction.
- some single celled organisms and almost all multicellular organisms also have a second type of reproduction in which the DNA molecules of two separate individuals are mixed together, producing an offspring that is genetically unique and different from siblings or parents. This is called sexual reproduction.
- multicellular organisms have life cycles that include a period during which DNA molecules, and the information they carry, are copied, mixed together in new combinations and then used to produce new individuals.
Biological information is stored in the linear sequence of bases in molecules of DNA. DNA molecules are one of many components in larger structures, called chromosomes that store, regulate, help express, replicate and transmit biological information.
- bacterial (prokaryotic) chromosomes consist of a single closed-circular loop of DNA packaged into membrane bound structure called a nucleoid.
- under favorable conditions the bacterial cells continuously replicate their DNA and, on average, each cell contains two copies of the DNA molecule.
- eukaryotic chromosomes consist of single linear DNA molecules which are packaged with proteins into a highly ordered complex called chromatin.
- histones are the major protein components of chromatin. These proteins are small, positively charged and bind tightly to DNA.
- nucleosomes are the basic units of chromosome structure. They consist of eight core histones in complex with 146 base pairs of DNA which is wrapped 1.8 times in a left-handed spiral around the proteins.
- chromatin is organized into higher levels of structure including fibers called solenoids, and at the highest level of structure into a protein maintained nuclear matrix.
DNA Replication: The Beginning
At the appropriate time in the cell cycle the replication of the DNA molecules within the chromosomes begins. The two complementary strands of the molecule are separated and each strand acts as a template for the synthesis of an opposite, new, complementary strand.
- DNA replication begins with the separation of the two complementary DNA strands that make up the molecule.
- the point at which the separation occurs and new synthesis begins is called the replication fork.
- replication always begins at the same point(s), called origins and proceeds in two directions away from the origins to the terminus.
- bacterial chromosomes usually have a single origin and a single terminus.
- eukaryotic chromosomes have multiple origins and termini. These replicons eventually fuse as replication takes place and the new chromosomes are synthesized.
Replication: At the Fork
At the replication fork DNA synthesis takes place using nucleotide triphosphates as precursors for each new strand. Enzymes line up a nucleotide base in a complementary relationship with one on the template strand, remove two of the phosphate groups from the nucleotide, and unite it with the new, growing strand.
- at each replicating fork there are two, separated, single strands of DNA acting as templates.
- new strands of DNA are made by joining together nucleotide bases one at a time.
- each new nucleotide is first paired with a base on the opposite complementary strand A with T and G with C.
- one of the new strands is made in one continuous length as the replicating fork moves away from the origin.
- the second new strand is made in a series of short, discontinuous lengths of DNA which are eventually joined together to form one long strand.
- DNA synthesis always needs a short piece of RNA synthesis to get it started. These short lengths of RNA are called primers.
Apart from one or two unique viruses, all DNA in all cells is replicated and synthesized in the same manner. The separation of the two complementary strands is followed by the creation of two new complementary strands.
- once the replication process is complete, each new daughter DNA molecule is half-old and half-new.
- in each new daughter molecule half the old molecule is preserved or conserved.
- this type of DNA replication is thus called semi-conservative.
Separation of the Chromosomes
Before a cell can divide into two new cells, the daughter DNA molecules must be identified and separated from each other in an unambiguous way that ensures that both new cells receive one copy each.
- in prokaryotic cells, which divide by binary fission, the chromosome is attached to the cell membrane. After DNA replication the two new copies are separated by the synthesis on new membrane between them.
- in eukaryotic cells, chromosomes are housed in the nucleus, where most of their regulation and replication takes place.
- prior to cell division the cells enter a period in which the chromosomes and chromatin condense and become highly packaged structures, visible under the microscope after staining with colored dyes.
- within these highly condensed chromosomal structures are two chromatids that hold identical copies of the DNA molecule.
- the two chromatids are held together by a centromere.
- protein microtubules attach to the centromere. These microtubules originate at opposite poles of the cell where they are anchored.
- as the microtubules attached to the centromeres shorten the chromatids separate and each is pulled to opposite sides of the cell.
- in this way the DNA molecules, and the copies of the biological information they carry, are also separated.
- the cell then divides along a line between the two sets of information, forming two new, identical daughter cells.
Science at a Distance
© 1997, 1998, 1999, 2000 Professor John Blamire