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A Beginners' Guide to Molecular Biology

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The chromosomes

Definition and description

A chromosome is the visible state of genetic material during a phase of the division of the cell (metaphaseask Dr Chromo!). Humans have 23 pairs of chromosomes, which makes the diploid ask Dr Chromo! number 46. The diploid number is the number of chromosomes of a normal cell. The haploid number is the number of chromosomes in a gameteask Dr Chromo!. Usually, the diploid number is twice the haploid number. The number of chromosomes varies depending on the species.

Just before mitosis,ask Dr Chromo! DNA, which contains the genetic information, is replicated. Now there are two identical copies of all the DNA in this cell. The cell is ready to divide.

chromosome fibre...

The DNA molecule is twisted onto itself, and the supercoiledask Dr Chromo! molecule is wrapped around proteins which maintain its shape. These proteins have a special name - histoneask Dr Chromo! proteins. The fibre has the structure of a spring (a solenoidask Dr Chromo!, to be technical), as shown in the illustration on the right. The DNA is wrapped around the protein, and then, twisted together again in the shape of a spring. The complex DNA protein is called chromatin, while the structure formed by two turns of DNA around one histone is called a nucleosome.
From DNA to chromosome

The LOOP, drawn on the right, is a well packed chromosome fibre. The chromosome fibre is coiled around a protein scaffold. The string of little packets is then wrapped around another protein structure to form a chromatidask Dr Chromo!.

chromosome A model of a chromosome during metaphase is displayed on the left:

A chromosome like this one is made of two identical molecules of DNAask Dr Chromo! efficiently packed into chromatids. As can be seen in the illustration, a chromatid is made of a protein frame, and loops around it.

The efficiency of this packing is pretty impressive! Human chromosome 1 is hardly 10 micrometers long. But it contains 7 centimetres of DNA!

DNA, DNA... what is DNA? Its structure? Its role?

chromosomes carry genes... and some concepts can be a bit complex. Chromosomes, genes, genotypes and other concepts are explained elsewhere...

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