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Facebook Twitter Email Print. It is known as nuclear DNA. In addition to nuclear DNA, a small amount of DNA in humans and other complex organisms can also be found in the mitochondria. They include two distinct groups: bacteria and archaea. Eukaryotes can be single-celled or multicellular. Eukaryotes encompass a wide range of organisms, from fungi to plants and animals.

How long is your DNA?

Most of the genetic material in most prokaryotes takes the form of a single circular DNA molecule, or chromosome. In addition, many prokaryotes also contain small circular DNA molecules called plasmids. These are distinct from their chromosomal DNA and in specific environments can provide certain advantages such as resistance to antibiotics. Nuclear DNA is organized into linear molecules called chromosomes.


  • Why does every cell in our body contain DNA? | Science Questions with Surprising Answers.
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  • DNA and the production of proteins.
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The size and number of chromosomes varies significantly between species. And once it knows it is a skin cell how does it know how to perform functions that skin cells need to do? And how does it know if it has been damaged by the sun and needs to be replaced by another skin cell? Every characteristic, quality, function, appearance and location of a cell is determined by DNA , which means that every characteristic, quality, function and appearance of our body is also determined by our unique DNA.

DNA is very complex and very tightly regulated chemical sequence that contains all the information our cells require to grow, perform functions and replicate. The information is contained in gene sequences , which are particular stretches of chemical patterns within the DNA that make up our genes. So firstly, what are genes?

Chromosomes - Pull up Those Genes

Genes carry our hereditary characteristics. For example one gene is responsible for our eye colour. This eye colour gene sits on the same part of DNA in every cell in the body however it is only the in the cells that make up the iris that that gene will be expressed. The same way that the gene that controls your heart beat will only be expressed in the heart, although the cells in the iris will also carry these genes. So you can imagine if each function of our cells and body is controlled by a separate gene then each of us needs a lot of genes.

The Human Genome Project set out to determine just how many genes there are in our DNA and what these genes are responsible for. In the first draft of the Human Genome Project was released; 30, genes were identified! Knowing where genes exist within the millions of chemicals that make up DNA has helped to characterise genetic defects and diseases and how these diseases can be treated or cured. The genome is important in every aspect of cellular function, including cell structure, growth, movement and division. The same set of DNA is found in the nucleus of every cell in our body.

The DNA is so tightly coiled and packed that it is estimated the nucleus of each human cell can hold about 2 meters of DNA. The DNA exists in 46 separate segments within the nucleus; these segments are known as chromosomes.


  • What is DNA??
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  • Extracting DNA from living things?
  • DNA is stored in chromosomes.

Each chromosome has a partner that contains the same genes in almost the same sequence of DNA, one member of the chromosome pair or homolog came from the father and one came from the mother during fertilisation. The chromosomes are what carry genes. An example of the genes carried by chromosomes are the sex genes , how the chromosome combination determines which gender you will be will be discussed later on. Information on re-publishing of our images.

The shape of DNA at the molecular level is thought to look like a gently twisting ladder.

Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include:. The chemical bond between the nucleotides always exists between A and T and G is always bound to C. For example;. Chemical bonds exist between strand 1 and strand 2 that connect the nucleotide pairs The strands are complimentary to one another, by knowing the sequence to one you can figure out the other because of the specific chemical bonds.

The very long DNA strands are coiled very tightly into chromosomes. The sperm contains half the amount of chromosomes that exist in other cells of the body and so does the egg. These pairs are referred to as homologs. The homologs contain a very similar DNA sequence and therefore they contain the same set of genes. However the paternal chromosome that contains the hair structure gene will contain an alternative version of this gene than the maternal chromosome; one may be a curly hair gene whereas the other chromosome may contain a straight hair gene.

An allele is the name given to one of these alternative sets of genes. Cross-over occurs where segments of the one chromosome are replaced with the corresponding segment from its homolog and vice versa. Once all 46 chromosomes have paired up and cross-over has occurred then the cell divides. Cells divide for a number of reasons. Firstly cells must divide in order to create life. As cells continue to divide they become the different cells in our body through a process called cell differentiation which will be discussed later on.

For example our skin cells have quite a rapid life cycle and are continuously being replaced by new cells. Cells need to be continually replaced because all cells have a life cycle and they will eventually die. When cells divide they undergo a process called mitosis. Each chromosome is duplicated so that the cell contains two lots of DNA. These strands are then duplicated nucleotide for nucleotide by another enzyme, DNA polymerase. Every chromosome is replicated in this manner so the cell will contain double copies of all of its DNA. Cells are continuously replicating and dividing.

This process is called protein synthesis.

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There are 64 possible combinations; for example. Each of these codons code for a specific amino acid that is floating around freely within the nucleus. Amino acids are the very small molecules that make up proteins. It is the tRNA molecule that binds these amino acids together as they are matched to the codon. Note there are only 20 amino acids and 64 codons so more than one codon matches each amino acid. When a cell grows, communicates with other cells, excretes waste, absorbs water, moves, sticks to other cells or dies and much more this is all due to the work of these proteins.

As we know, different cells have different functions so not all cells will have the same proteins, yet all cells have the same DNA. This leaves the question; how does one type of cell make different proteins from another type of cell? For different cells, different parts of the booklet are read and this is what differentiates one type of cell from another. For example our blood cells will read one chapter of this huge book.

Where is DNA found in Cell?

The next chapter may be read by kidney cells and the next by liver cells and so on. So every cell type will transcribe and translate different genes that are contained in the DNA. Genetic disorders or diseases are caused by particular genes or chromosomes abnormalities , sometimes these are inherited but they can also occur spontaneously during reproduction. Disjunction ensures each daughter cell receives its own copy of that chromosome.

Non-disjunction occurs when the chromosomes fail to seperate resulting in one daughter cell with one too many of the chromosome and one daughter cell with one too little.

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What is DNA? - Genetics Home Reference - NIH

This can happen with the sex chromosomes or any of the other chromosomes. As we discussed earlier, the sex of a growing embryo is determined by the combination of genes passed on from our parents. When the egg divides one X chromosome is given to each of the daughter cells. The sperm then enters the egg carrying either an X or a Y chromosome. If the sperm is carrying an X, the cell will contain two X chromosomes and will become a female.