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Biology - Chromosomes

We will now talk a little about chromosomes. Chromosomes are made of deoxyribonucleic acid which is called DNA. Chromosomes are found in the nucleus of every cell throughout the entire body. Human beings have a total of 46 chromosomes but these chromosomes exist in pairs. So, sometimes we say that human beings have 23 pairs of chromosomes. Think for a second about one pair of chromosomes the two members of any recognized pair of chromosomes are similar but not exactly alike. The two members of a recognized pair of chromosomes are called homologous chromosomes. 

If we took the 23 pairs of chromosomes from a human cell and then from each cell we removed one number of the pair will be left altogether with 23 different chromosomes and this cell would be called haploid. When we say haploid, we mean a cell for which the chromosomes are not paired. A normal human cell that has 23 pairs of chromosomes is said to have a diploid number of chromosomes and the cell is therefore called a diploid cell. Now let us talk a little about DNA. DNA is made of little subunits called nucleotides. 

A nucleotides is made up of pentose sugar attach to a phosphate and to a nitrogenous base. A nucleotide can have as its base anyone of the following four possible bases; adenine, guanine, cytosine, and thymine. One strand of DNA is formed when many nucleotides bond together in one base order or another and form a chain. A chromosome can be thought of as two strands of DNA put together side by side. Each stand is said to be the compliment of the other. 

A chromosome looks like a ladder the sides of the ladder are made up of the phosphate sugar components of all of the nucleotides. Each wrung of the ladder is made from a pair of bases. One base comes from a nucleotide on one strand of DNA and the other base comes from nucleotide on the other strand of DNA. By convention, when we list the sequence of nucleotides in a nucleotide strand we read from the five prime to the three prime end. When two strands of DNA get together to form a chromosome, the wrungs are formed by the bonding of two bases. But it turns out that each base is only willing to get together with one of the other bases. You must remember that adenine bonds with thymine and guanine bonds with cytosine. 

There are two hydrogen bonds between adenine and thymine and three hydrogen bonds between guanine and cytosine. If you know the base sequence of one strand you can figure out the base sequence of its compliment. Suppose we tell you that in one area of a DNA molecule, one of the strands has the base sequence adenine, cytosine, thymine and guanine by knowing the base pairing rules you can figure out the base sequence of the complimentary strands, which is thymine, guanine, adenine, and cytosine. One other thing you should know is that two of these bases are called Purines and two are called Pyrimadines. 

It is easy to remember which are which. Remember out alphabetized list of nucleotides bases adenine is a purine, cytosine is pyrimadine, guanine is a purine and thymine is a pyrimadine. Notice that the first one is a purine and that the terms pyrimadine and purine alternate along the rest of the list. You will also notice that each purine bonds with one of the pyrimadines and likewise each pyrimadine bonds with one of the purines. Purines do not bond with purines and pyrimadine do not bond with pyrimadines. When the double stranded DNA molecule is fully formed it is twisted. The twisted ladder shape is called a double helix. Watson and Crick are the scientist who figured out the shape of DNA. And that is why the double-helix shape is sometimes called the Watson Crick model of DNA or the Watson Crick double helix. 

It is easy to see how DNA reproduces its self. The first thing that happens is that using DNA helices enzymes the double helix unwinds and unzipped so that the two strands of DNA can separate. Remember DNA synthesis always proceeds in a five prime to three prime direction. DNA synthesis begins at a particular site called the origin of replication. An RNA primer is used to add subunits to the three prime end of the DNA strand. 

The next thing that happens is that with help of the enzymes DNA polymerase the nucleotides line up according to the rules of base pairing. Cytosine’s line up next to guanines and guanines line up next to cytosines, adenines line up next to thymines and thymines line up to next to adenines. Along side of each separated strand nucleotides lines up and form a new second strand both strand are replicated at the same time. One strand continuously called the leading strand and the other strand discontinuously called the lagging strand. 

Bonds formed between the base pairs and among the sugar phosphate components of the nucleotides. An organism’s chromosomes contain all of its genetic information. In terms of what you are born with chromosomes make you who you are. They do that basically by determining what proteins your cell will manufacture. Since proteins often serves as enzymes chromosomes determine what enzymes your cells manufacture. 

Enzymes determine what chemical reactions will and will not occur in your cells and that’s what makes you more or less who you are. DNA directs protein synthesis through RNA. RNA looks like very much like DNA but there are three important differences between them. RNA is single stranded. The nucleotides that form RNA have ribose instead of deoxyribose as their sugar. In fact, RNA stands for ribonucleic acid and DNA stands for deoxyribonucleic acid. RNA nucleotides never contain thymine as a base instead they contained a base called uracil. 

So the four bases found in RNA nucleotides are adenine, cytosine, guanine, and uracil. RNA is formed under the direction of DNA. Sometimes a DNA molecule will unzipped when it is not planning to replicate and one of the strands will serve as template for the formation of an RNA molecule. The RNA molecule is formed using the enzyme RNA polymerase, usually one strand is transcribed. The process can be induced or suppressed depending on the needs of the cell. In RNA molecule is formed just like a DNA molecule except that you think of uracil instead of thymine. 

If a DNA strand has the sequence guanine, thymine, adenine and cytosine the RNA molecule that it forms will have the sequence cytosine, adenine, uracil, and guanine. The order of nucleotides and the RNA molecule is determined by the order of nucleotides on the DNA strand that produces it. If a different DNA strand had served as template, a different RNA molecule would have resulted. The process of making RNA molecules from DNA is called transcription. 

Since RNA is formed from chromosomes, it is formed in the nucleus. However, after it is formed it moves out into the cytoplasm. RNAs are important because they are involved in protein synthesis. There are three types of RNAs. First there is mRNA which id formed in the nucleus from a DNA template. This is the RNA that goes out into the cytoplasm to locate itself on ribosome’s it called messenger-RNA because it carries a message from the nucleus to the cytoplasm, second is tRNA. TRNA stands for transfer-RNA. 

The tRNA reads the mRNA codon and brings the appropriate amino acid into line. The third RNA is called rRNA. RRNA stands for ribosomal-RNA. Ribosomal-RNA is made in the nucleolus and it makes a part of the ribosome. Every three RNA nucleotides bases are called a codon. Each codons specifies an amino acid, each codons are like a message from the chromosomes directing the ribosomes to produce a certain proteins. The three base sequence on the tRNA molecule is called an anti-codon. The anti-codon can recognize a codon on the mRNA by complimentary base pairing. Translation is the process in which the amino acids line up in accordance with the codons that appears on a molecule of RNA. 

The ribosome has subunits a large one and a small one. The ribosome also has two special tRNA binding sites called the A site and P site. A special tRNA called methanol tRNA is the initiator of protein synthesis. It attaches at one end to methionine and to a sequence of three nucleotide bases at the other end. This initiator binds to the ribosome at the P site. The P stands for peptidyl tRNA binding site. Once initiation is complete the amino acids are ready to added one by one to the initial amino acid which is methionine. The next amino acid is carried by a TRNA a peptide bond is formed between the two amino acids after the peptide bond is formed methionine is now attach to the tRNA at the A site. 

The tRNA is released from the P site. The tRNA at the A site carries the growing polypeptide. Next the tRNA in the A site jumps to the P site. This is called translocation. This process allows for the next tRNA to come in and attach to the next amino acid. This translocation process requires energy which is provided by the hydrolysis of GTP. The mRNA moves through the ribosome in the five prime to three prime direction. Chromosomes are the template from which mRNA is formed. That means that the base sequence in an mRNA molecule is a reflection of the base sequence of the DNA molecule that served as its template. 

The mRNA goes out into the cytoplasm and there each of its three base sequences or codons attracts the molecule of tRNA with a complimentary three-base sequence or anti codon. The tRNA molecule is attached to a particular amino acid for which it is specific. The amino acids thus line up in an order dictated by the order of the codons on the mRNA molecule. Now remember this, chromosomes are DNA molecules. These DNA molecules give rise to complimentary mRNA molecules. A molecule dictates the order in which amino acids get together to form proteins. Proteins serves as enzymes and enzymes determine what chemical reaction occur in our bodies.