Bethlehem is the Middle Eastern city where
Jesus was born.
BRITANNICA STUDENT ENCYCLOPEDIA Christianity 113
Christianity in the Middle Ages
Christianity continued to gain strength
during the Middle Ages, a period from
about 500 to 1500. Its leaders became
powerful and wealthy, and the church
was the chief cultural institution in
Europe.
But as Christianity expanded, it did not
remain unified. In 1054 the Eastern
church at Constantinople separated
from theWestern church in Rome
because of differences in beliefs and
practices. This split created the Eastern
Orthodox churches and the Roman
Catholic church.
Another major development was the
Crusades. In the 11th century a Muslim
Turkish empire conquered southwestern
Asia, including many places associated
with Jesus. Over the next two centuries,
armies of European Christians fought
Muslim forces in the Middle East, partly
in an attempt to recapture the Holy
Land.
Modern Period
In the 1500s a German priest named
Martin Luther began to question
certain practices and teachings of the
Roman Catholic church. Luther’s
criticism helped bring about the
religious revolution known as the
Reformation. As a result, various
reformers eventually founded the first
Protestant churches.
As Christianity flourished in Europe,
Christians came to believe that their
religion should be taught to people in
other lands. Beginning in the 1500s and
continuing into the 21st century, Christian
missionaries brought their faith to
parts of Africa, Asia, and North and
South America. These efforts helped
make Christianity the most widespread
religion in the world.
#More to explore
Bible • Church • Crusades • Eastern
Orthodox Churches • Jesus Christ
• Judaism • Protestantism • Reformation
• Roman Catholicism
Christmas
Marking the birth of Jesus, Christmas is
a major religious holiday for Christians.
For many people it is no longer only a
religious festival. It has become a season
of gift giving and celebration. The date
of Christmas is December 25. But
Christmas celebrations often take place
over several days or weeks.
In addition
to teaching
about their
religion, missionaries
often
set up schools
and medical
clinics.
Girls carry candles and sing in
front of a festive Christmas tree
in Seoul, South Korea.
114 Christmas BRITANNICA STUDENT ENCYCLOPEDIA
Several traditional customs are part of
the Christmas holiday in the United
States. Decorating a Christmas tree with
strings of lights and ornaments is one
beloved custom. Another common custom
is gift giving. This is frequently
associated with a figure named Santa
Claus.
In many European countries families
exchange gifts on Christmas Eve,
December 24. Like Santa Claus, Father
Christmas and Pere Noel are said to
deliver presents to children in the
United Kingdom and France, respectively.
In Spain and Italy children receive
presents on the night of January 5.
As part of the religious holiday, many
Christians attend church services on
Christmas Eve or Christmas Day. The
word Christmas comes from the Old
English term Cristes maesse, meaning
“Christ’s mass.” That was the name of
the Christian worship service held on
December 25 to honor the birth of
Jesus. Christians believe that Jesus was
born in Bethlehem. The actual day and
year of his birth are not certain.
#More to explore
Bethlehem • Christianity • Jesus Christ
Chromosome
Chromosomes are tiny, threadlike structures
inside every cell of every organism,
or living thing. Chromosomes carry
information about the organism in units
called genes. When living things reproduce,
they pass their genes along to their
offspring.
General Features
In animals and plants, chromosomes are
located inside the nucleus, or central
structure, of each cell. A material called
deoxyribonucleic acid (DNA) makes up
most of each chromosome. Plant and
animal chromosomes also contain a
material called ribonucleic acid (RNA)
and some proteins.
Some organisms, including bacteria, do
not have a nucleus. Their chromosomes
float freely within the cell. In these
organisms, chromosomes are made up of
only DNA.
Chromosome Numbers
Every living thing has a specific number
of chromosomes in most of its cells. In
most plants and animals, chromosomes
come in pairs. Potato cells have 24 pairs,
for a total of 48 chromosomes. Mosquito
cells have three pairs, for a total of
six chromosomes. Human cells have 23
pairs, for a total of 46 chromosomes.
Damage to
chromosomes
can cause
birth defects
and such
diseases as
cancer.
Chromosomes are inside the cells of every
living thing. They are so small that they can
only be seen through a powerful microscope.
BRITANNICA STUDENT ENCYCLOPEDIA Chromosome 115
Humans and most other animals have
the same number of chromosomes in
every type of cell except one—the sex
cells. These are the egg cells produced by
females and the sperm cells produced by
males. The sex cells have only one half
of each pair of chromosomes.
During reproduction, the father’s sperm
cell and the mother’s egg cell combine
to form a new cell. This cell is the beginning
of an embryo, or developing baby.
Each sex cell brings its half set of chromosomes
to the embryo. In this way, the
