Chromosomes (1)

Question 1

What are chromosomes?

Answer 1

Chromosomes are long, thread-like structures that form part of the chromatin network in the nuclei of cells. They are made up of a strand of DNA wound around proteins called histones.

Question 2

Chromosomes were discovered by chance in ____. They absorb dye very easily which is why they were called chromosomes - coloured bodies. This makes them visible under a microscope but they can only be seen as individual threads when a cell is dividing.

Answer 2

1888

Question 3

In somatic (body) cells of diploid organisms:

• The number of chromosomes in each cell is the___.
• The chromosomes are made up of two sets; one chromosome of each pair comes from the mother (___ chromosome) and one comes from the father (____ chromosome). They are therefore called diploid cells (di - two) or 2n for short.
• For each paternal chromosome, there is a matching maternal chromosome forming a _____ pair. The chromosomes forming a pair are the same size and shape and have the same genes in the same place, but the ____ for each trait may not be the same.

Answer 3

same.  
(maternal chromosome) 
(paternal chromosome)
homologous pair
alleles

Question 4

How does DNA divide?

Answer 4

The DNA of each chromosome replicates to form two identical threads or chromatids joined by a centromere. This takes place in the interphase of a cell cycle, i.e. between cell divisions. These threads only become visible when they shorten and thicken as a cell divides.

Question 5

Why is DNA replication important?

Answer 5

Replication of DNA is very important to ensure that, as a cell divides, each daughter cell receives a full complement of all the genetic material.

Question 6

What is meant by chromosome number?

Answer 6

Each species has a specific number of chromosomes in its somatic cells. Some organisms have identical chromosome numbers but these need not be related. It is the similarities in the DNA of the chromosomes that show relationships, not their number.