Vocabulary: genes, DNA, replication, mutation, gamete, heredity, complementary base pairing, chromosomes, asexual, mitosis, crossing over, genetic recombination, natural selection, cancer, adenine, guanine, cytosine, thymine, template, RNA, mRNA, rRNA, tRNA, transcription, regulation
All Organisms have a set of instructions that determine their characteristics. These instructions are called genes and contain the instructions for life that are passed from parents to offspring during reproduction.
The inherited instructions that are passed from parent to offspring exist as a code. The DNA molecule which makes up our genes contains this code.
Asexual v. Sexual Heredity
The DNA molecules must be accurately replicated before being passed on. Asexually reproducing organisms normally pass on this genetic code identically between the parent and offspring, while the offspring of sexual reproduction produce offspring that resemble their parents, but exhibit some variations from them.
Changes in DNA or mutations which occur in non sex cells of a sexually reproducing organism will not be passed on to their offspring. Mutations which occur in sex cells or gametes will be frequently be passed on to their offspring.
Once the coded information contained in the DNA molecule is passed on, it is used by a cell to make proteins. The proteins that are made become cell parts and carry out most functions of the cell. The subtle differences in DNA between different human beings and different species results in the production of different proteins. This is a major reason why we show individual differences.
DNA Structure and Function
DNA provides the set of coded instructions required by every organism for specifying its traits. The DNA molecule also provides for a reliable way for parents to pass their genetic code from one generation to the next. Heredity refers to this passage of these instructions from one generation to another.
DNA is a double stranded molecule which has the shape of a twisted ladder. This shape is called an alpha helix. The sides of this twisted ladder are composed of alternating phosphate and deoxyribose sugar units, while the rungs of the ladder are composed of pairs of nitrogenous bases. These bases are called adenine (A), thymine (T), guanine (G), and cytosine (C). These bases exist in pairs on the rungs of the ladder with A always pairing with T and G pairing with C. This principle is sometimes called complementary base pairing. (The saying G CAT provides a means of remembering this idea.)
Structure of the DNA molecule
Location of DNA
Hereditary information is contained in genes, which are composed of DNA, located in the chromosomes of each cell. Chromosomes are found in the nucleus of each cell.
The Gene Chromosome Model
Each gene carries a separate piece of information. An inherited trait of an individual can be determined by one genes, but is usually determined by the interaction of many different genes.
A single gene can influence more than one trait. A human cell contains many thousands of different genes coding for many different traits. Changes in the sequence of the DNA molecule and therefore the gene are called mutations. A mutation may change the manner in which a trait is expressed by an organism.
Every organism requires a set of coded instructions for specifying its traits. For offspring to resemble their parents, there must be a reliable way to transfer information from one generation to the next. Heredity is the passage of these instructions from one generation to another. The DNA molecule provides the mechanism for transferring these instructions.
In asexually reproducing organisms, all the genes come from a single parent. As asexually produced offspring are produced by the cell division process of mitosis, all offspring are normally genetically identical to the parent.
In sexually reproducing organisms, the new individual receives half of the genetic information from its mother through the egg and half from its father from his sperm. Sexually produced offspring resemble, but are not identical to, either of their parents. Some reasons for these variations between sexually reproduced offspring and their parents include crossing over when gametes are formed in each parent and genetic recombination, which is the combining of the genetic instructions of both parents into a new combination in the offspring when fertilization occurs.
Note that two of the four offspring in the punnett square at the right have a completely different genetic makeup than that of either parent
The processes of crossing over and genetic recombination will result in offspring exhibiting variation from the original parents. The variations shown between different sexually produced offspring provide the driving force for the process of natural selection.
Heredity and Environment
The characteristics of an organism can be described in terms of combinations of traits. Traits are inherited, but their expression can be modified by interactions with the environment. Examples of this include the lack of color in completely shaded grass, even though it still possesses the genetic makeup to appear green and the change in fur color of returning fur in a shaven Himalayan hare at cold temperatures.
Effect of Cold on Himalayan Hare Fur Color
The application of an ice pack to a region of shaved hair results in black hair growing back instead of the original white color.
The many body cells in an individual can be very different from one another, even though they are all descended from a single cell and thus have identical genetic instructions. This is because different parts of these instructions are used in different types of cells, influenced by the cellís environment and past history. Poor health habits can have an adverse effect on the development and expression of many genes in human cells, resulting in sickness or even death.
A mutation is a change in the genetic material of an organism.
Mutations which occur in non sex cells of sexually reproducing organisms will not be passed on to the offspring, although they may result in disease or death for the organism involved. One possible consequence of a mutation in a non sex cell is uncontrolled mitotic cell division or cancer.
Mutations which occur in sex cells or gametes may be passed to the offspring. Along with crossing over and genetic recombination, mutation provides for a source of variation in sexually reproducing individuals.
In all organisms, the coded instructions for specifying the characteristics of the organism are carried in DNA. The genetic code is contained in the four nitrogenous bases of DNA; adenine, guanine, cytosine, and thymine. These bases are often indicated only by using their beginning letters A, G, C, and T. Each individual DNA strand serves as a template or model for the formation of other DNA molecules by replication.
DNA codes for the formation of RNA in the nucleus of the cell. RNA is short for another kind of nucleic acid called ribonucleic acid. RNA is very similar in structure to DNA except for three small differences. These differences include the fact that RNA is a single stranded molecule, lacks the base thymine (T) as it is replaced by the base uracil (U), and its five carbon sugar ribose has one more oxygen atom than the sugar in DNA. Three different types of RNA exist, mRNA or messenger RNA, tRNA or transfer RNA, and rRNA or ribosomal RNA.
Cells store and use coded information. The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires. The chemical and structural properties of DNA are the basis for how the genetic information that underlies heredity. DNA is encoded in the sequence of nitrogenous bases which directs the formation of proteins in the cell. How does this process work? First, the DNA code is copied on to the mRNA (messenger RNA) codon. A codon is a sequence of three nitrogenous bases. This process is called transcription. This mRNA codon is then carried from the nucleus out to the ribosome. Messenger RNA attaches to another kind of RNA called tRNA (transfer RNA). Transfer RNA attaches to amino acids and carries them to the ribosome. This assembly of amino acids due to the code provided to RNA by the original DNA molecule is what produces proteins for the cell. Remember a protein is a long molecule formed from amino acid subunits.
In summary, the code of DNA directs the synthesis of RNA, which in turn directs the making of proteins on the ribosomes. This is sometimes referred to as being the central dogma or idea of biology. There are 64 possible combinations of triplets (sequences of 3 nitrogenous bases) which code for the 20 different possible amino acids. As the DNA of different organisms and most individuals (except for identical twins) is different, this means the proteins produced by different humans and other organisms exhibit differences. It is these differences which make us unique individuals.
The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. Protein molecules are long, usually folded chains made from 20 different kinds of amino acids in a specific sequence. This sequence influences the shape of the protein. The shape of the protein, in turn, determines its function.
Offspring resemble their parents because they inherit similar genes (DNA sequences) that code for the production of proteins that form similar structures and perform similar functions.
Cell functions are regulated. Regulation occurs both through changes in the activity of proteins and through the selective expression of individual genes, as humans and other organisms have genes which direct the expression of other genes. This regulation allows cells to respond to their environment and to control and coordinate cell growth and division.
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