01. Formation of Chromatin

02. Histones and Chromatin

03. Chromosomes

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Within the chromosomes, chromatin proteins such as histones compact and serve to package the DNA and control its functions.. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

Eukaryotic organisms (animals, plants,fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria orchloroplasts.

In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm.

Typically,eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule.

The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated. Unduplicated chromosomes are single linear strands, whereas duplicated chromosomes contain two identical copies (called chromatids or sister chromatids) joined by a centromere.

Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure (pictured to the right) if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends. Chromosomal recombination plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may unexpectedly evade apoptosis leading to the progression of cancer.

Perspective: In practice "chromosome" is a rather loosely defined term. In prokaryotes and viruses, the term genophore is more appropriate when no chromatin is present. However, a large body of work uses the term chromosome regardless of chromatin content. In prokaryotes, DNA is usually arranged as a loop, which is tightly coiled in on itself, sometimes accompanied by one or more smaller, circular DNA molecules called plasmids. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest genophores are found in viruses: these DNA orRNA molecules are short linear or circular genophores that often lack structural proteins.[citation needed]

Some species also contain plasmids or other extrachromosomal genetic elements.

 

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Function

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Deoxyribonucleic acid (DNA) is the nucleic acid involved in the genetic code. DNA sequences contain the genetic instructions used in the development and functioning of all known living organisms except RNA viruses.

It is the sequence of these four nucleobases along the backbone that encodes genetic information used in the development and functioning of all known living organisms except RNA viruses. The DNA sequences have structural purposes and are also involved in regulating the use of genetic information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription.

Heredity

DNA packs in all the genetic information and passes it on to the next generation. DNA holds the instructions for an organism's or each cell’s development and reproduction and ultimately death.

This is accomplished throguh the process of DNA replication

In this process the DNA strands, that are tightly wound with each other, unwind and literally unzip to leave several bases without their partners on the other strand and remain along the backbone of the molecule.

The bases are very specific about which base they will attach to and the adenine only pairs with thymine and guanine will only pair with cytosine. Unpaired bases come and attach to these free bases and a new strand is formed that is complementary to the original sequence.

The end result is a strand that is a perfect match to the original one prior to it unzipping. This result in two new pairs of strands and two coiled DNA. Each of the new DNA contains one strand from the mother pair and a new one.

This is used in in reproduction, maintenance and growth of cells, tissues and body systems.

Making Protein

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It is the sequence of these four nucleobases along the backbone that encodes genetic information used in the development and functioning of all known living organisms except RNA viruses. The DNA sequences have structural purposes and are also involved in regulating the use of genetic information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription.

Although DNA contains the genetic blueprint of life, it requires the assistance of ribonucleic acid (RNA) to be functional. After DNA is converted into strands of RNA, the messenger RNA (mRNA) is sent to the ribosome to direct the synthesis of proteins.

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The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription which specifies the sequence of the amino acids within proteins.

Within cells DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes.

Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA inorganelles, such as mitochondria or chloroplasts.[1] In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

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For more information on DNA read the following:

1. http://en.wikipedia.org/wiki/DNA

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Animated Graphic DNA Structure: http://www.johnkyrk.com/DNAanatomy.swf ( John Kyrk Master of Biology, Harvard and Animator.)

Conccccccccccccctent 3

a. Transcription

The first step that occurs is a process known as transcription. DNA is read by the (who does this) messengers that break it open into single stranded polynucleotide chains and is copied into RNA. RNA acts as a messenger to carry the information to other parts of the cell.

RNA forms opposite bases from that present on the DNA. For example, G on the DNA forms C on the RNA strand.

Each of the bases gets together in threes and these form particular amino acids. There are 20 such amino acids. These are also known as the building blocks of proteins.

The amino acids first form a long chain called the polypeptide chain. This polypeptide chain undergoes conformational and structural changes and folds and refolds over itself to form the final complex structure of the protein.

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b. Translation

The next step is translation. In this step the cell organelles called ribosomes come into play. These ribosomes act as translators by translating the messenger's code into the proper protein format or a chain of amino acids that form the building blocks of the protein. Proteins are created by ribosomes translating mRNA into polypeptide chains. Each amino acid is formed by combining three bases on the RNA.

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c. Posttranslational modification (PTM)

This is the third step before sending it to the required areas in the body. These polypeptide chains undergo PTM, (such as folding, cutting and structuring), before becoming the mature protein product.

Posttranslational modification of insulin. At the top, the ribosome translates a mRNA sequence into a protein, insulin, and passes the protein through the endoplasmic reticulum, where it is cut, folded and held in shape by disulfide (-S-S-) bonds. Then the protein passes through the golgi apparatus, where it is packaged into a vesicle. In the vesicle, more parts are cut off, and it turns into mature insulin.

A protein (also called a polypeptide) is a chain of amino acids. During protein synthesis, 20 different amino acids can be incorporated to become a protein. After translation, the posttranslational modification of amino acids extends the range of functions of the protein by attaching it to other biochemical functional groups (such as acetate, phosphate, various lipids and carbohydrates), changing the chemical nature of an amino acid (e.g. citrullination), or making structural changes (e.g. formation of disulfide bridges).

Also, enzymes may remove amino acids from the amino end of the protein, or cut the peptide chain in the middle. For instance, the peptide hormone insulin is cut twice after disulfide bonds are formed, and a propeptide is removed from the middle of the chain; the resulting protein consists of two polypeptide chains connected by disulfide bonds. Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid. This amino acid is usually taken off during post-translational modification.

Other modifications, like phosphorylation, are part of common mechanisms for controlling the behavior of a protein, for instance activating or inactivating an enzyme.

Post-translational modification of proteins is detected by mass spectrometry or Eastern blotting.

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Human cells normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell.

Humans have 23 pairs of chromosomes – a total of 46 chromosomes. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair is called the sex chromosomes and differs between males and females. Females have two copies of the X chromosome or XX, while males have one X and one Y chromosome.

Both parents have reproductive cells – sperms in fathers and ovum or eggs in mothers. These sperms and eggs contain half the number of chromosomes – 23 each. When the egg and the sperm fertilizes, this gives rise to a cell that has the complete set. Thus a person inherits half of his or her genes from each of the parents.

Human cells normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell.

Humans have 23 pairs of chromosomes – a total of 46 chromosomes. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair is called the sex chromosomes and differs between males and females. Females have two copies of the X chromosome or XX, while males have one X and one Y chromosome.

Both parents have reproductive cells – sperms in fathers and ovum or eggs in mothers. These sperms and eggs contain half the number of chromosomes – 23 each. When the egg and the sperm fertilizes, this gives rise to a cell that has the complete set. Thus a person inherits half of his or her genes from each of the parents.

 

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