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Summary DNA chromosomes
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HC3 DNA Chromosomes (BOOK)Chapter 4 pp 173-187 (+ Fig. 4-61)
DNA, Chromosomes, and Genomes
Hereditary information is the genome (the complete store of information in an organism’s DNA) of
an organism that is passed on from a cell to its daughter cell at cell division, and from one generation
to the other. The information is stored within every living cell as genes which are information
containing elements.
Hereditary information is carried on chromosomes which are the carriers of genes. Chromosomes
consist of deoxyribonucleic acid (DNA) and protein, both present in the same amount.
We now know the sequence of the 3.2 billion nucleotide pairs that provide information for producing
a human adult from a fertilised egg.
The structure and function of DNA
A deoxyribonucleic acid (DNA) molecule consists of two long polynucleotide chains composed of four
types of nucleotide subunits. These chains are known as DNA chains or DNA strands and they are
hold together with hydrogen bonds between the base portions. In DNA, the sugar deoxyribose is
attached to a single phosphate group (deoxyribonucleic acid) and a base which may be adenine (A),
cytosine (C), guanine (G) and thymine (T).
The way nucleotides are linked together gives the DNA strand a chemical polarity. This polarity in a
DNA chain is indicated by referring to one end as the 3’end and the other as the 5’end.
The 3’end has a hydroxyl and the 5’end has a phosphate.
The three-dimensional structure of DNA, called
the DNA double helix, arises from the chemical and structural features of its two polynucleotide
chains. There is one complete turn every 10 base pairs.
The members of the base pairs (A-T and C-G) can fit together within the double helix only if the two
strands are antiparallel, this means both the 3’end and 5’end not on the same end.
Mechanism for heredity
DNA can carry information in a chemical form because DNA is in fact a linear polymer of four
different kinds of monomers, strung out in a defined sequence like the letter of a document written
in alphabetic script.
, Another thing is that DNA can carry this information from the one generation to the other. This is
because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the
nucleotide sequence of its partner strand so each strand can act as a template for the synthesis of a
new complementary strand.
Organisms differ from one another because their respective DNA molecules have different nucleotide
sequences and therefore carry different messages.
DNA in eukaryotes
Nearly all the DNA in eukaryotic cells is sequestered in the nucleus. The nucleus is delimited by a
nuclear envelope which is formed by two concentric lipid bilayer membranes. These membranes are
at some spots punctured by large nuclear pores through which molecules can move.
The nuclear envelope is directly connected to the endoplasmic reticulum.
Just beneath the inner nuclear membrane there is a thin felt like mesh called the nuclear lamina.
Chromosomal DNA and its packaging in the chromatin fiber
Eukaryotic DNA
Each chromosome in a eukaryotic cell consists of a single DNA molecule along with the proteins that
fold and pack the DNA into a more compact structure. The complex of DNA and tightly bound protein
is called chromatin.
Except gametes and a few highly specialised cells, each human cell nucleus contains two copies of
each chromosome, one inherited from the mother (maternal) and one from the father (paternal).
The maternal and paternal chromosomes are called homologous chromosomes (homologs). The
only non homologs are the sex chromosomes in males (X and Y).
The display of the 46 human chromosomes at mitosis is called the human karyotype.
Chromosomes contain long strings of genes
A gene is often defined as a segment of DNA that contains the instructions for making a particular
protein but this is only a part of a gene’s function. Genes that code for a protein are indeed the
majority, but there are many so called RNA genes that generate a functionally significant RNA
molecule instead of a protein as their final product.
NOTE!
There is no simple relationship between chromosome number, complexity of the organism, and total
genome size.
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