CELL FUNCTION (DNA, RNA, and Protein Synthesis)

    1. Cell division is the process by which cells reproduce themselves. It consists of nuclear (karyokinesis) division and cytoplasmic division (cytokinesis). This type of division which results in two identical (diploid) cells is collectively called mitosis. Cell division which results in cells which are haploid (that is having ½ the normal chromosome number) is called meiosis. Meiosis requires two actual cell divisions. During the first anaphase of meiosis the paired homologus chromosomes are separated. During the second anaphase chromatids are separated. The sole purpose of meiosis is the creation of gametes (sex cells - Spermatazoa and ova).
    2. A cell carrying on every life process except division is said to be in interphase or metabolic interphase. During this time prior to karyokinesis and cytokinesis, the DNA molecules, or chromosomes, replicate themselves so the same chromosomal complement can be passed on to future generations of cells. In interphase the DNA material is called chromatin. DNA is assisted in “unzipping” for replication by means of a group of enzymes collectively referred to as DNA Polymerase. DNA assembles RNA (transcription) with assistance of a group of enzymes collectively referred to as RNA Polymerase.
    3. The nitrogen base pairs of DNA are held together by weak chemical bonds called hydrogen bonds. A nitrogen base plus a sugar and a phosphate is called a nucleotide. The sugar found in the DNA molecule is called deoxyribose sugar. The sugar found in the RNA molecule is called ribose sugar. It is from the types sugars that the respective molecules are named.
    4. Mitosis is the distribution of two sets of chromosomes into separate and equal nuclei following their replication. It consists of prophase, metaphase, anaphase, and telophase. During prophase the nuclear membrane dissolves freeing up the chromosomes. Each chromosome consists mainly of two tightly coiled strands of DNA called chromatids. These are joined at some place along their length by a centromere. Each is attached to a spindle fiber.
    5. Genes are sequences of nitrogen bases along each DNA molecule. For each chromsome there is a single spindle fiber attached at the centromere. The mitotic spindle is formed between two structures called centrioles. Spindle fibers are made from structures called microtubules which are made of protein (tubulin). Human beings have 23 pairs of chromosomes in normal body cells. A mature chromosome is made up of two strands of DNA along with associated support proteins. Paired genes which occur on paired chromosomes are called alleles. Gametes (spermatozoa and ova) in human beings have 1/2 the normal number of chromsomes. We call this condition haploid. Cells with the normal number of chromosomes are referred to as being diploid.
    6. During metaphase the chromosomes line up along the center or equator of the cell. During anaphase each chromosome is pulled apart separating the chromatids with each moving to the opposite ends of the cell. During telophase cytokinesis which begins in late anaphase terminates. A cleavage furrow forms at the cell's equator and progresses inward, cutting through the cell to form two separate portions of cytoplasm. A new nuclear membrane forms in each new cell thus creating two cells in the place of the original. These are sometimes called daughter cells. These new cells immediately move into the interphase condition. Plant cells differ from animal cells in telophase by the formation of a cell wall section called the cell plate. This occurs instead of cytokinesis because of the presence of the plant cell wall.
    7. Deoxyribonucleic acid (DNA) is a nucleic acid which carries genetic instructions for the biological development of all cellular forms of life and many viruses. It is sometimes referred to as the master molecule of heredity as it is inherited and used to perpetuate traits. Prior to the process of reproduction, it is replicated and distributed to the gametes (spermatozoa and ova) then by their union passed on to offspring.
    8. In bacteria and blue green algae (prokaryotic organisms), DNA is distributed more or less throughout the cell. In the complex (eukaryotic) cells of plants, animals and in other multi-celled organisms, most of the DNA is found in the chromosomes, which are located in the cell nucleus. Chloroplasts and mitochondria (cellular endosymbionts) also carry DNA, as do many viruses.
    9. Genes are the organism's blueprint. A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function which we currently don’t understand. Structurally DNA is organized as two complementary strands with weak hydrogen bonds between them that can be "unzipped" like a zipper. The DNA code is made up of four interchangeable nitrogen bases. They are: Adenine, Thymine, Guanine, and Cytosine. Each base links with only one other base: A+T, T+A, C+G and G+C. The order of the bases along the length of the DNA is what it's all about, the sequence itself is the prescription for genes.
    10. DNA replication prior to cell division is performed by splitting (unzipping) the double strand down the middle and recreating the "other half" of each new single strand by attracting complimentary nucleotides from the nuclear cytoplasm. Since each of the four nitrogen bases can only combine with one other base, the base on the old strand dictates which base will be on the new strand. This way, each split half of the strand plus the bases it attracts end up as a complete replica of the original. On occasions mutations may occur. Mutations are simply chemical imperfections in this process: a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to. Mutations are either spontaneous or induced. A DNA strand is really a pair of molecules, which entwine to form a double helix. Thus each helix is a chemically linked chain of nucleotides, each of which consists of a sugar, a phosphate and one of the four kinds of nitrogen bases The diversity of the bases means that there are four kinds of nucleotides, which are commonly referred to by the identity of their bases. These are adenine (A), thymine (T), cytosine (C), and guanine (G) nucleotides. Each base forms hydrogen bonds readily to only one other -- A to T and C to G.
    11. Within a gene, the sequence of nucleotides along a DNA strand defines the production of a protein, which a cell manufactures using the information of the sequence. When DNA copies itself prior to cell division the copying process is known as Replication. When DNA is used as a pattern during the manufacture of RNA the process is known as Transcription. Finally when RNA manufactures protein the process is known as Translation. It should be noted here that RNA differs from DNA in two significant ways. First in RNA the nitrogen base Thymine is replaced by Uracil, and second the sugar is different. In DNA the sugar is deoxyribose sugar. In RNA the sugar is ribose sugar. The genetic code is made up of three letter words formed from a sequence of three nucleotides for example: ACU, CAG, or UUU. These codons are located on each strand of messenger RNA. Transfer RNA also bear base sequences called anticodons. Transfer RNA molecules each attracts a particular amino acid. Since there are 64 possible codons, most amino acids have more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region.
    12. DNA replication or DNA synthesis is the process of copying the double-stranded DNA prior to cell division. The two resulting double strands are generally almost perfectly identical, but occasionally errors in replication can result in a less than perfect copy called a mutation. Each of them consists of one original and one newly synthesized strand. This is called semiconservative replication. The process of replication consists of three steps: initiation, replication and termination. The hydrogen bonds between the strands of the double helix are weak enough that they can be easily separated by enzymes. Enzymes known as helicases unwind the strands to facilitate the advance of sequence-reading enzymes such as DNA polymerase. The unwinding requires that helicases chemically cleave the phosphate backbone of one of the strands so that it can swivel around the other
    13. In the 19th century, biochemists initially isolated DNA and RNA (mixed together) from cell nuclei. They realized only later that nucleotides were of two types--one containing ribose and the other deoxyribose. It was this subsequent discovery that led to the identification and naming of DNA as a substance distinct from RNA. Friedrich Miescher (1844-1895) discovered a substance he called "nuclein" in 1869. Later, he isolated a pure sample of the material now known as DNA from the sperm of salmon, and in 1889 his pupil, Richard Altmann, named it "nucleic acid". This substance was found to exist only in the chromosomes. In 1944, the renowned physicist, Erwin Schrödinger, published a brief book entitled What is Life? , where he maintained that chromosomes contained what he called the "hereditary code-script" of life. Francis Crick, James D. Watson, Maurice Wilkins, Rosalind Franklin, Seymour Benzer , et al., took up the physicist's challenge to work out the structure of the chromosomes and the question of how the segments of the chromosomes that were conceived to relate to specific traits could possibly do their jobs. In the 1950s, only a few groups made it their goal to determine the structure of DNA. These included an American group led by Linus Pauling, and two groups in Britain. A key inspiration in the work of all of these teams was the discovery in 1948 by Pauling that many proteins included helical (see alpha helix) shapes. Watson and Crick had begun to think about double helical arrangements which they modeled. Watson and Crick's model attracted great interest immediately upon its presentation. Arriving at their conclusion on February 21, 1953, Watson and Crick made their first announcement on February 28. Their paper 'A Structure for Deoxyribose Nucleic Acid' was published on April 25. In an influential presentation in 1957, Crick laid out the "Central Dogma ", which foretold the relationship between DNA, RNA, and proteins. Work by Crick and coworkers deciphered the genetic code not long afterward. These findings represent the birth of molecular biology. Watson, Crick, and Wilkins were awarded the 1962 Nobel Prize for Medicine for discovering the molecular structure of DNA
    14. Cells of the human body regularly manufacture over 100,000 different proteins such as - eye, hair and skin color, hormones, structural proteins, etc. DNA (deoxyribonucleic acid) is referred to as nucleic acid. One strand of DNA has a backbone consisting of a polymer of the simple sugar deoxyribose bonded to a phosphate. The backbone of a strand of DNA resembles this: sugar-phosphate-sugar-phosphate-sugar- etc.. Each sugar molecule in the strand also binds to one of four different nucleotide bases. These bases: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T), are the beginnings of a molecular alphabet. Each sugar molecule in the DNA strand will bind to one nucleotide base. Each strand of DNA contains millions or even billions (in the case of human DNA) of nucleotide bases. These bases are arranged in a specific order according to our genetic ancestry. The order of these base units makes up the code for the specific characteristics in the body a whole. As there are 26 letters in the English alphabet arranged in various sequences to produce words, our body's DNA uses 4 letters (the 4 nucleotide bases) to code for millions of different physical characteristics.
    15. Each molecule of DNA is actually made up of 2 strands of DNA cross-linked together. Each nucleotide base in the DNA strand will cross-link (via hydrogen bonds) with a nucleotide base in a second strand of DNA forming a structure that resembles a ladder. These bases cross-link in a very specific order: A will only link with T (and vice-versa), and C will only link with G (and vice-versa). In 1953, James Watson, Francis Crick and Rosalind Franklin discovered that the structure of DNA is actually a double helix. In other words, the DNA ladder described above coils around itself somewhat like a doubled spring. A strand of DNA may be millions, or billions, of base-pairs long A Gene is a relatively small segment of DNA that codes for the synthesis of a specific protein. Minimum size is three base pairs called a triplet. This protein then will play a structural or functional role in the body. A chromosome is a larger collection of DNA that contains many genes and the support proteins needed to control these genes.
    16. Protein synthesis is a 2 part process that involves a second type of nucleic acid along with DNA. This second type of nucleic acid is RNA, ribonucleic acid. RNA differs from DNA in two respects. First, the sugar units in RNA are ribose as compared to DNA's deoxyribose. Because of this difference, RNA does not bind to the nucleotide base Thymine, instead, RNA contains the nucleotide base Uracil (U) in place of T (RNA also contains the other three bases: A, C and G). Transcription: In the first step of protein synthesis, the 2 DNA strands in a gene that codes for a protein unzip from each other. Similar to the way DNA replicates itself, a single strand of messenger RNA (mRNA) is then made by pairing up mRNA bases with the exposed DNA nucleotide bases. Remember that mRNA does not contain the base Thymine, so U is paired with each of DNA's A bases.
    17. Translation: After the mRNA is manufactured, it leaves the cell nucleus and travels to a cellular organelle called the ribosome (we will learn about the cell, nucleus and ribosome in the next lesson). In the ribosome, the mRNA code is translated into a transfer RNA (tRNA) code which, in turn, is transfered into a protein sequence. In this process, each set of 3 mRNA bases (the mRNA base triplet is called a codon) will pair with a complimentary tRNA base triplet (called an anticodon). Each tRNA is specific to an amino acid, as tRNA's are added to the sequence, amino acids are linked together by peptide bonds, eventually forming a protein that is later released by the tRNA. Proteins normally consist of hundreds or thousands of amino acids.Synthesis of a protein requires the participation of a ribosome, ATP, mRNA, tRNA, and a releasing factor.
    18. Mutations are caused by various mutagenic agents (chemicals, x-rays, U.V. light, etc.). Most but not all are deleterious because the amino acid sequences of proteins and nucleotide sequences of DNA have been determined during millenia of natural selection. Mutations also result in changes in the order of nucleotides in genes. A point mutation is the substitution of one nucleotide for another. In a frameshift mutation , a nucleotide is added or deleted. A specific example concerns causation of a type of skin cancer in humans. Ultraviolet light causes a mutation in human skin cells (two adjacent cytosines are converted into thymines). The result is squamous-cell carcinoma.