Chromosomes, Genes and DNA

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Way back in the dark ages when I went to high school, we where taught that humans had 32(?) chromosomes and that each chromosome contained individual genes which was what actually passed on characteristics and traits like eye color etc. So now there is DNA and the humane genome is billions of bits, links, whatever long.

So how does DNA relate to chromosomes and genes? I understand that genes are actually segments of the DNA molecule; but, what is the relationship if any between chromosomes and DNA molecules? Does each chromosome contain one complete molecule of DNA or are chromosomes completely out of the picture now? (Yet they still talk about X and Y chromosomes and genes carried on or within or attached to them.)
 

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  • #2
iansmith
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There's 23 chromosome in human.

Here some definition that might help you


Gene

The basic unit of heredity; the sequence of DNA that encodes all the information to make a protein. Structurally, a gene is formed by three regions: a regulatory region called the promoter juxtaposed to the coding region containing the protein sequence, and a "3’ tail" sequence. In mammalian cells, the promoter is a complex region containing binding sites for many proteins that regulate gene expression. A gene may be "activated" or "switched on" to make protein – this activation is referred to as gene expression - by these proteins which control when, where and how much protein is expressed from the gene. In the human genome, there are an estimated 30,000 to 40,000 genes. Some of these are evolutionarily related and form "gene families" that express related proteins. There are also genes that no longer make a protein; these defective remnants of evolution are called pseudogenes.

DNA (Deoxyribonucleic Acid)

The chemical that forms the basis of the genetic material in virtually all living organisms. Structurally, DNA is composed of two strands that intertwine to form a spring-like structure called the double helix. Each strand is formed by a backbone of deoxyribose sugar molecules linked by phosphate residues. Attached to each backbone are chemical structures called bases, which protrude away from the backbone towards the center of the helix, and which come in four types - Adenine, Cytosine, Guanine, and Thymine (designated A, C, G and T). In DNA a C can only hydrogen-bond with a G, and an A only with a T, these interactions, formed by so called hydrogen bonds, hold the two strands together. Each strand of DNA has a series of Gs, As, Ts and Cs attached to its backbone. It is the sequence of these bases that forms the code which is translated by cellular machinery to create a new protein. The other (complementary or antisense) strand always has a sequence that matches the first strand, with each C complemented by a G, and each A by a T, and vice versa.

Chromosome

One of the physically separate segments that together form the genome, or total genetic material, of a cell. Chromosomes are long strands of genetic material or DNA that have been packaged and compressed by wrapping around proteins. The number and size of chromosomes varies from species to species. In humans, there are 23 pairs of chromosomes (a pair has one chromosome from each parent). One pair are called the sex chromosomes because they contain genes that determine sex. The chromosome carrying the male determining genes is designated Y and the corresponding female one the X chromosome. The remaining pairs are called autosomes. Chromosome 1 is the largest and chromosome 22 the smallest. Each chromosome has two "arms" designated p and q.

http://www.geneed.com/glossary/index.html
 
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Okay, thanks for the information, but my question remains partially unanswered. Does each chromosome contain one whole seperate strand of DNA or is there just one strand in each cell that are seperate bundled packages of segments of the one DNA strand per cell? Is there one strand or are there 23 seperate strands one for each chomosome per cell?
 
  • #5
Moonbear
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Another way to think about it that might help...
You'll have 46 strands of DNA in each cell. Think of them as threads.
These threads pair up, so you have a total of 23 pairs of strands.
On each of these threads, you have many genes (think of them like a lot of knots tied in the thread...a poor analogy, but a good visual...or like multicolored thread, each segment of color is a gene).
Now, those long pieces of thread get wrapped around spools (the spools are histones) to condense the unruly threads into a smaller and neater arrangement.
This entire combination of paired threads wrapped around spools is called a chromosome. Since you had 23 pairs of threads to start, you have 23 chromosomes.

There are some times during meiosis, which forms eggs and sperm, when those pairs of threads separate and the single threads go to two different cells. In that case, the single thread is also called a chromosome.

One last thing to remember, because this usually trips people up. When you look at a piece of thread, you know how it's actually two or more threads intertwined? In this analogy, think of each DNA thread as actually containing two fibers (I'm using "fibers" to avoid re-using the term thread, and confusing the analogy further), those two fibers are the double helix structure you hear about.
 
  • #6
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Thank you, all of you. for the information. I think that I have a better understanding now. This has bothered men for sometime as I have never seen or read before the relationship between chromosomes and DNA.
 

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