do we have replicates of the same DNA molecule? or replicates of several DNA molecules?

[bobrilla]

I am trying to understand if a chromosome is made of one DNA molecule or just a part of a DNA molecule. Maybe i am confused with the DNA molecule itself. The way it is taught seems to indicate that each human beings has DNA molecules that are all the same. Then how can the same molecule make different chromosome with different size?

[Strange]

Each chromosome has a DNA molecule. (Or does it have two copies?) The DNA molecule in each chromosome can be a differnt size. Because DNA is a long chain (well, two chains) of nucleotides so a molecule can be of any length.

[bobrilla]

how can it be same molecule in each chromosome if it has different size and different number of genes ?

[Strange]

Well, it isn't the same molecule; they are all DNA but they are different molecules of DNA. You can think of DNA as a "template" for a molecule rather than a specific compound: it can be of any length and can have any pattern of bases making up the nucleotides. Each chromosome is of a specific length and has a sequence of bases that define the genes in the chromosome.

 

BTW. This is not my subject so I may get some details wrong...

Edited January 17, 2014 by Strange

[CharonY]

Each chromatid contains one DNA molecule. After the S-cycle an (eukaryotic) chromsome contains two identical chromatids. However between chromsomes the DNA varies significantly even within a single organism.

The basic structure of DNA is identiical in all living organisms. However, DNA consists of a series of nucleotides that have different bases. The number of nucleotides and the sequence of bases is what makes all the difference.

[bobrilla]

okay, thank you for your answers! what confused me at first was how this subject is taught at school and in books. Talking about THE DNA of someone is pretty confusing. Why not saying it is a group of molecules for example?

[CharonY]

I guess it depends on school, school year and curriculum. Oftentimes simplifications are made that are alright in the precise context but not in others. A precise way to refer to the genetic material of an organism would be genome, for example.

[ahyaa]

I am trying to understand if a chromosome is made of one DNA molecule or just a part of a DNA molecule. Maybe i am confused with the DNA molecule itself. The way it is taught seems to indicate that each human beings has DNA molecules that are all the same. Then how can the same molecule make different chromosome with different size?

 

The answer to your question is this: a CHROMOSOME is ONE LINEAR DNA MOLECULE. That is, a chromosome is an unbroken length of DNA. Therefore, a chromosome is not a "part" of a DNA molecule, it IS the DNA molecule, and it is made up of smaller building blocks called nucleotides. To help you envision how the structure is organized, here's a little "path" from smallest to largest (if you haven't learned some of these terms yet, then don't worry about them):

 

Nucleotides -> Codons -> Genes -> Chromosomes -> Genomes

 

To answer the second part of your question: each INDIVIDUAL human has its OWN UNIQUE GENOME. So, with the little "path" above in mind, if each individual human has own unique genome, then it has its own unique set of chromosomes! (Because a genome = set of chromosomes). Humans have 23 pairs of chromosomes. The 23 pairs of chromosomes ARE NOT IDENTICAL TO EACH OTHER, but you have the SAME 23 CHROMOSOMES in EACH CELL!

 

I hope that makes it clearer, I apologize for the caps if you don't like them, I just thought it'd help explain it more clearly. And someone brought up chromatids... do not worry about chromatids - they are not necessary in answering your question and may confuse you more. Chromatids are only of worry when you are dealing with replicated chromosomes.