Britannica Illustrated Science Library: Evolution and Genetics by Encyclopedia Britannica

Author:Encyclopedia Britannica [Britannica, Encyclopedia]
Language: eng
Format: epub
Published: 2011-10-29T23:50:27+00:00

58 MECHANISMS OF HEREDITY

EVOLUTION AND GENETICS 59

The Chromosome

2

3

6

THE FRAMEWORK

SOLENOID

The chromosome is a structural unit that consists of a molecule of DNA Each one of the rosettes

A group of six

associated with proteins. Eukaryote chromosomes condense during consists of loops

loops

nucleosomes that

mitosis and meiosis and form structures visible through a microscope.

stabilized by the

IN EACH ROSETTE

form each turn

“scaffolding” of other

inside the loops

They are made of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), and proteins. These

proteins. The majority of the proteins are histones, small positively charged loops help to

condense the

molecules. Chromosomes carry the genes, the functional structures chromatin.

responsible for the characteristics of each individual.

0.0000012 inch

(0.00003 mm)

DIAMETER OF EACH SOLENOID

Karyotype

The ordering and systematic classification of

the chromosomes by pairs, size, and position

of the centromere. The chromosomes

that are seen in a karyotype are

found in the metaphase of

mitosis. Each one of them

consists of two sister

chromatids united by

their centromeres.

SPACER DNA

The nucleosomes are

1

united by chains of

base pairs of DNA

0.0000004 inch

CHROMATINS

(0.00001 mm) long.

There are two types:

euchromatin, lightly

6

packed, and

heterochromatin, more

nucleosomes

densely packed. The

PEARL NECKLACE

majority of nuclear

IN EACH TURN

If the DNA chain is stretched

chromatin consists of

and observed under a

euchromatin.

microscope, it resembles

30

beads on a string.

Nevertheless, DNA chains are

60

rosettes

generally found pressed very

IN EACH TURN OF

tightly around the nucleus.

base pairs

THE SPIRAL

THE AMOUNT OF

DNA BETWEEN

NUCLEOSOMES

NITROGEN

BASES

4

CIRCULAR

CHROMOSOME

NUCLEOSOME

OF BACTERIA

A group of eight histone

molecules with two DNA

Carrier of Genes

spirals twisted around

them. The “tails” of the

In the DNA, certain segments of the molecule are

histones seem to interact

called genes. These segments have the genetic

PROKARYOTE CELL

with the molecules that

information that will determine the characteristics of an Prokaryote cells do not have a

regulate genetic activity.

individual or will permit the synthesis of a certain protein.

cellular nucleus, so the DNA is found

The information necessary for generating the entire organism in the cytoplasm. The size of the DNA

is found in each cell, but only the part of the information differs according to species.

necessary for reproducing this specific type of cell is Prokaryotes are almost all unicellular

activated. The reading and transmission of the information organisms belonging to the domains

for use outside the nucleus is performed by messenger RNA.

of the archaea and bacteria.

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