Organization of DNA into chromosomes

Within each nucleated human cell there are two complete copies of the genome. The genome is 'the haploid genetic complement of a living organism' and in humans contains approximately 3 200 000 000 base pairs (bp) of information, which is organized into 23 chromosomes. Humans contain two sets of chromosomes - one version of each chromosome inherited from each parent giving a total of 46 chromosomes (Figure 2.3). Each chromosome contains one continuous strand of DNA, the largest - chromosome

(a) Deoxynucleotide 5'-triphosphate

(b) Deoxyribose

(c) Nitrogenous bases

Base

J C1

J C1

Adenine (A)

Cytosine (C)

Figure 2.1 The DNA molecule is built up of deoxynucleotide 5'-triphosphates (2.1a). The sugar (2.1b) contains five carbon atoms (labelled C1 to C5); one of four different types of nitrogenous base (2.1c) is attached to the 1 prime (1') carbon, a hydroxyl group to the 3' carbon and the phosphate group to the 5' carbon

O h2c

O h2c

1 base pair 9

Adenine

'Ny-N Cytosine

Figure 2.2 In the DNA molecule the nucleotides are joined together by phosphodiester bonds to form a single stranded molecule (2.2a). The DNA molecule in the cell is double stranded (2.2b) with two complementary single stranded molecules held together by hydrogen bonds. Adenine and thymine form two hydrogen bonds while guanine and cytosine form three bonds

h2c h2c

'îi / \ - 1

3

5

11

Ht il 12

14

15

18

3 ft

B ii

i

i

19

20

21

22

X

Y

Figure 2.3 The male human karyotype pictured contains 46 chromosomes, 22 autosomes and the X and Y sex chromosomes - the female karyotype has two X chromosomes (picture provided by David McDonald, Fred Hutchinson Cancer Research Center, Seattle and Tim Knight, University of Washington)

Figure 2.3 The male human karyotype pictured contains 46 chromosomes, 22 autosomes and the X and Y sex chromosomes - the female karyotype has two X chromosomes (picture provided by David McDonald, Fred Hutchinson Cancer Research Center, Seattle and Tim Knight, University of Washington)

1 - is approximately 250 000 000 bp long while the smallest - chromosome 22 - is approximately 50 000 000 bp [2-4].

In physical terms the chromosomes range in length from 73 mm to 14 mm. The chromosomes shown in Figure 2.3 are in the metaphase stage of the cell cycle and are highly condensed - when the cell is not undergoing division the chromosomes are less highly ordered and are more diffuse within the nucleus. To achieve the highly ordered chromosome structure, the DNA molecule is associated with histone proteins, which help the packaging and organization of the DNA into the ordered chromosome structure.

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