Can anyone help me with my science?

Question

I need help with the following questions:

1. why is DNA considered a helical molecule?

2. why are cells considered alive but molecules are not?

Please help me out. Thanks.

Answer 1

Molecular structure

Although sometimes called "the molecule of heredity", DNA macromolecules as people typically think of them are not single molecules. Rather, they are pairs of molecules, which entwine like vines, in the shape of a double helix (see the illustration at the right).

DNA consists of a pair of molecules, organized as strands running start-to-end and joined by hydrogen bonds along their lengths. Each strand is a chain of chemical "building blocks", called nucleotides, of which there are four types: adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). (Thymine should not be confused with thiamine, which is vitamin B1.) The DNA of some organisms, most notably of the PBS1 phage, have Uracil (U) instead of T.

Each strand of DNA is a covalently linked chain of nucleotides, with alternating sugar ( deoxyribose)- phosphates forming the "backbone" for the nucleobases ("bases"). The negatively-charged phosphate groups between each deoxyribose make DNA an acid in solution and allow DNA molecules of different sizes to be separated by electrophoresis. Because DNA strands are composed of these nucleotide subunits, they are polymers. The major difference between DNA and RNA is the sugar, 2-deoxyribose in DNA and ribose in RNA.

Base pairing

DNA is composed of 4 bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Uracil (U), is rarely found in DNA except as a result of chemical degradation of Cytosine, but the DNA of some viruses (notably PBS1 phage DNA) and RNA (Ribonucleic Acid), has Uracil instead of Thymine.

Each base on one strand forms a bond with just one kind of base on another strand, called a "complementary" base: A bonds with T, and C bonds with G. Therefore, the whole double-strand sequence can be described by the sequence on one of the strands, chosen by convention. Two nucleotides paired together are called a base pair.

In a DNA double helix, two polynucleotide strands can associate through the hydrophobic effect and pi stacking. Which strands associate depends on complementary pairing. Each base forms hydrogen bonds readily to only one other base, A to T forming two hydrogen bonds, and C to G forming three hydrogen bonds. The GC content and length of each DNA molcule dictates the strength of the association; the more complementary bases exist, the stronger and longer-lasting the association, characterised by the temperature required to break the hydrogen bond, its melting temperature (also called Tm value)).

A cell's machinery separates the DNA double helix, and uses each DNA strand as a template for synthesizing a new strand which is nearly identical to the previous strand. Errors that occur in the synthesis are called mutations. Mutations are the results of the cells' attempts to repair chemical imperfections in this process, where a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to. On rare occasions, wrong pairing can happen, when thymine goes into its enol form or cytosine goes into its imino form. Mutations can also occur after chemical damage (through mutagens), light (UV damage), or through other more complicated gene swapping events. This process of replication is mimiced in vitro by a process called Polymerase chain reaction (PCR).

.

.

.

.

.

.

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is sometimes called the building block of life.[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an ostrich egg. In 1837 before the final cell theory was developed, a Czech Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[2]

The word cell comes from the Latin cellula, meaning, a small room. The descriptive name for the smallest living biological structure was chosen by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.[3]

.

Molecule (1794) - "extremely minute particle," from Fr. molécule (1678), from Mod.L. molecula, dim. of L. moles "mass, barrier". A vague meaning at first; the vogue for the word (used until late 18th century only in Latin form) can be traced to the philosophy of Descartes.

Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom. The term 'molecule' in gases has been used as a synonym for the fundamental particles of the gas, whatever their structure. This definition results in a few types of gases (for example inert elements that do not form compounds, such as neon), which has 'molecules' consisting of only a single atom.[6]

Answer 2

Google it instead of wasting your time coming here and asking.