Chromosomes and DNA

4. 1. Chromosome act as the storage facility of
genetic information
2. They are responsible for the expression of
characters
3. They are passed on from parent to offspring and
thus they transmit hereditary information from
one generation to the next
4. Chromosome are subject to change; thus they
are also factors of variation

5.  Chromosomes are found in the nucleus of a
eukaryotic cell.They are chemically made up of
molecules of protein and DNA.

6.  They are thread-like structures in cells at interphase,
the stage in the cell cycle when cells are “at rest”
from division.

7.  Double-stranded chromosomes become tightly
coiled into rod shaped structures and are distributed
equally to daughter cells during cell division.

8.  After a cell divides by mitosis, the two daughter cells
produced contain single-stranded chromosomes, the
number of which is exactly the same as the
chromosome number of the parent cell.

9.  When the cell divides by meiosis, the four daughter
cells produced contain single-stranded
chromosomes, the number of which is reduced to
half the chromosome number of the parent cell.

10.  Cells need to go through synthesis phase of
interphase after division, to allow the duplication of
chromosomes and to return single-stranded
chromosomes back to double-stranded state.

11.  Mitosis occurs in most body cells (also called somatic
cells). Most body cells are diploid, represented by 2N,
which means they contain two sets of chromosomes.

12.  Meiosis occurs during sexual reproduction to produce
sex cells or gametes. Gametes (sperm and egg) are
haploid, represented by N, which means they contain
only one set of chromosomes.

13.  All cells come from pre-existing cells.
 Cell division is the process that results in the
formation of new cells.
 Cells division, generally, involves two parts:
1. Karyokinesis - the division of the nucleus
2. Cytokinesis - the division of the cytoplasm

14.  A chromosome in a cell that is at rest from
division is “double stranded”. Each strand is
called a chromatid.

15.  Since the two chromatids in a chromosome con
tain identical genes or genetic information, they
are also sometimes referred to as sister
chromatids. Each sister chromatid is:
1. basic proteins called histones
2. nucleic acid called deoxyribonucleic acid or DNA

16. How do these structural and chemical
properties of the chromosome help in
the performance of its functions?

17. DNA is the major chemical component of
chromosomes. DNA is replicated during
synthesis phase of interphase before a cell
divides.

18. DNA replication provides the mechanism
for chromosome duplication. Single-
stranded chromosomes can be duplicated
to produce double-stranded chromosomes.

19. Sister chromatids contain identical genetic
information; they separate to form single-
stranded chromosomes, and are equally
distributed to daughter cells during
division.

20. Thus, cell division involves not only the
equal distribution of chromosomes to the
nucleus of the daughter cells, but also the
transmission of genetic information from
parent to daughter cells, from one
generation to the next.

21. Moreover, since the chromosomes are
made up of DNA, and DNA is the genetic
material, chromosomes act as the storage
facility of genetic information and carry the
code for the expression of inherited
characters.