MOLECULAR GENETICS II

Chargaff's rules

 

    The race to unveil the structure of DNA was guided by Chargaff's rules. No model

    could be taken seriously if it violated any of these following guidelines.

 

    1. In DNA, the number of pyrimidines equals the number of purines

                    that is   C + T = A + G

 

    2. A = T

        C = G

 

    3. The A/T and C/G ratios varied among species but were consistent within a species.

 

    The model proposed by Watson and Crick (read the original article here) obeys these

    rules. In fact, in their paper to Nature, they write "It has not escaped our notice that

    the specific pairing we have postulated immediately suggests a possible copying

    mechanism for the genetic material". It's been called the understatement of the

    century.

 

    DNA packaging

 

    DNA in prokaryotes is "naked"; that is, the molecule is free of any associations with

    any other type of molecule. In eukaryotes, the DNA is complexed with proteins in a

    very structured manner to form discrete units of heredity in the nucleus called

    chromosomes. In growing cells, the DNA/protein complex is more properly termed

    "chromatin". When cells enter a division phase, the material condenses and can be seen

    clearly as chromosomes. These chromosomes can be "picked out" of the cell and used to

    form karyotypes.

 

    DNA is packaged in a systematic manner to facilitate regulation and expression. In the

    first level of folding, the DNA molecule is wrapped around a core of eight histone

    proteins and capped with another histone, H1, to form a nucleosome. When the entire

    length of the molecule is wrapped this way, the resulting structure resembles "beads

    on a string". The nucleosomes are then wrapped tightly to form a 30 nm fibre called a

    solenoid.  The fibres are packaged into looped domains (third level) and then finally

    placed on non-histone protein scaffolds. At metaphase, the duplicated chromosome has

    a width of 500-700 nm. Remember that DNA itself is only 2 nm wide.

 

    Architecture of a chromosome

 

GREAT SITE (before I forget) plus one more

 

    Chromosomes are more easily seen in a dividing cell. Chemical agents can stop this

    process to view the duplicated metaphase chromosomes or to take them from the cell

    for analysis (see karyotype) . The two chromatids of the duplicated chromosome

    are identical copies of each other. Each chromatid will be passed onto a daughter cell

    when cell division is completed. They are held together at a restricted region called the

    centromere. The centromere is sandwiched between proteins called kinetochores that

    attach to the spindle fibres. Extending from the centromere are two arms. The short

    arm is termed "p", and the long arm is termed "q".

    When geneticists speak of a specific region on a chromosome, they will use locators

    such as 15q11-q13. This is the region in chromosome 15 that is deleted in the

    Prader-Willi syndrome.

 

    The ends of the chromosomes are called telomeres and have been associated with

    limitations on the numbers of cell divisions (Hayflick limit) and aging.

 

    Generally, to count chromosomes, one should count centromeres not chromatids.

 

    The concept of ploidy

 

    Ploidy refers to the number of sets of chromosomes in a given cell or an organism.

    Humans have two sets of chromosomes - one from each parent. That makes our cells

    diploid. Our gametes - sperm and ova - contain one set of chromosomes to be passed

    onto our children: these cells are haploid.  The endosperm in the seeds of angiosperms

    contains three sets and is, therefore, triploid.  Bread wheat is a hexaploid and

    strawberries are octoploid.

 

    Ploidy is symbolized by the letter N. In humans, N=23. This is the number of

    chromosomes in one set. In our body (somatic) cells, 2N = 46.

 

    Cell cycle

http://www.cellsalive.com/cell_cycle.htm

 

    Each cell has a life cycle that is determined by its function, location, and complexity.

    Brain cells last throughout our lifetimes, never dividing, and never being replaced if

    damaged beyond repair. Many nerve cells behave this way as well. Much research is

    being done to coax these cells (especially spinal cord nerve cells) to divide and repair.

 

    Cells alternate between periods of growth and work (interphase) and cell division.

    Interphase is the longest period of the cell cycle and can be broken into three stages.

    The G1 phase is the phase when the cell grows and fulfills its function. It is followed by

    the S phase in which the genetic material is duplicated (think "S" for DNA synthesis).

    In the final phase, the G2 phase, the cell completes its preparations for cell division.

    These stages are identical whether the cell is destined to undergo mitosis or meiosis.

    Cells that are not destined to divide again are said to be in a G₀ phase.

 

    After the G2 phase, the cell will undergo a division phase: mitosis or meiosis.

    Mitosis creates two identical daughter cells and maintains the ploidy. Any eukaryotic

    cell regardless of ploidy or numbers of chromosomes is capable of mitotic division.

    Mitotic division describes the division of the nucleus. Many times, but not always,

    mitosis is followed by cytokinesis, which is the division of the cytoplasm and its

    constituents.