This document provides information about genetics and DNA structure. It explains that DNA is packaged into chromosomes inside the nucleus of cells. DNA is made up of nucleotides that form genes, which determine traits. Genes are segments of DNA that code for specific proteins and physical characteristics. Chromosomes have two structures - loose chromatin and tightly coiled chromatids, with DNA wrapped around histone proteins in each structure.

1. Grade 9 Quarter 1
Heredity: Inheritance and Variation
Lesson: Location of Gene in the Chromosome
(DNA in a Chromosome)

2. Objectives:

3. Genetics
Is a branch of Biology concerned with the study of genes, genetic
variation and heredity in organisms.

4. DNA (deoxyribonucleic acid)
- The cell is composed of many organelles, and
one of these organelles is the Nucleus.
- The Nucleus is the storehouse of DNA inside
the cell protecting the DNA at all times from
damage. Since there are molecules that may
damage the DNA, the structure of the nucleus
has a special membrane called the nuclear
envelope that encloses the DNA.
- The DNA is packaged by a special group of
proteins form tightly coiled structures
referred to as chromosomes.
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5. A DNA molecule consists of
two long strands wrapped
around each other to form a
double helix.
The double helix looks like a
twisted ladder, and each
strand is composed of many
repeating units called
nucleotides.

6. Nucleotides
- Nucleotides form genes, and a single DNA molecule may contain thousands of
genes. Genes determine all hereditary traits, and they control all the activities that
take place within cells.
- Each nucleotide has three
parts:
• Nitrogen base
• Sugar
• Phosphate group

7. Genes
A specific gene is encoded by the sequence of nitrogen bases along a
strand of DNA.

8. The genetic language, however, uses an alphabet with only four letters
– the four kinds of nitrogen bases in DNA.

9. These segments of the DNA strand code for functional products or
physical traits, characteristics, physical structure and functions.

10. An average gene will
contain 1,000 of these
four bases and can be
arranged in 4,000
different ways depending
on the required genetic
code needed for a certain
gene.

11. 2 Types of Chromosome
Structure:
• Chromatin
- A loose loop of DNA
strand that has a “beads-
on-a-string” structure.

12. • Chromatin
- Each “bead’ consists of
double-stranded DNA
wrapped twice around a
core of eight proteins
called Histones which
help organize the coiling
and folding of DNA.

13. • Chromatid
- The coiled and folded loops of the
DNA strand condense even more.
- A pair of chromatids held together
at the center by centromere
constitutes a Chromosome.