DNA

What is DNA?

• DNA = Deoxyribonucleic Acid

• DNA is the basis of our inherited traits.

• DNA is a sugar found mostly in the cell nucleus.

• DNA makes up our genes and chromosomes.

• DNA is found mostly in the nucleus, but some is also found in other organelles.

Why is DNA stable?

• DNA contains deoxyribose sugar.

• Compared with ribose, deoxyribose has one less oxygen.

• This makes DNA more stable and less likely to mutate/break down.

🧬 DNA & Differences Between People

• About 99.9% of human DNA is identical between people.

• The remaining 0.1% contributes to differences between individuals, such as:

  • Hair colour

  • Eye colour

  • Height

  • Other inherited traits

Structure of DNA

DNA has a double helix structure — it looks like a twisted ladder.

DNA is made of:

1. Sugar

2. Phosphate

3. Nitrogen bases

Four nitrogen bases:

Base-pairing rule:

A ↔ T
G ↔ C

This is because the bases have complementary shapes and bonding sites that allow them to fit together properly.

• A + T → 2 hydrogen bonds

• G + C → 3 hydrogen bonds

Sugar-phosphate backbone

• The sugar and phosphate form the sides of the DNA ladder.

• The base pairs form the rungs.

• The sugar in DNA is specifically deoxyribose, which is a 5-carbon sugar.

Easy way to remember:qaSZ7;/

A-T = 2
G-C = 3

And:

Sides = sugar + phosphate
Middle = base pairs

2. Structure of DNA

DNA has a double helix structure.

Think of it like a twisted ladder:

• The sides of the ladder = sugar + phosphate

• The rungs = pairs of nitrogen bases

DNA is a polymer, meaning it is made from many smaller building blocks called nucleotides.

Each nucleotide contains 3 things:

1. Deoxyribose sugar

2. Phosphate group

3. Nitrogenous base

Nucleotide = sugar + phosphate + nitrogeno us base

3. The Four Nitrogen Bases

There are four bases in DNA:

• A = Adenine

• T = Thymine

• C = Cytosine

• G = Guanine

Base-pairing rules

A pairs with T

C pairs with G

Easy way to remember:

A-T = 2 hydrogen bonds
C-G = 3 hydrogen bonds

The bases pair this way because their shapes and hydrogen-bonding sites are complementary.

Hydrogen bonds

• A–T → 2 hydrogen bonds

• C–G → 3 hydrogen bonds

The hydrogen bonds help hold the two DNA strands together.

4. DNA's Backbone

The sugar and phosphate molecules alternate to form the backbone of DNA.

The nitrogen bases point toward the middle and form the rungs.

Remember:

Backbone → sugar + phosphate

Rungs → nitrogen base pairs

5. DNA's Double Helix

A DNA molecule consists of two strands that are twisted together into a double helix.

The structure helps make DNA stable because:

• The two strands are held together by hydrogen bonds between complementary bases.

• The bases fit together specifically.

• The sugar-phosphate backbone provides structural support.

6. DNA → Genes → Chromosomes

These three terms are closely connected.

DNA

The molecule containing genetic information.

Gene

A specific segment of DNA containing instructions for a particular function/trait, often involving making a protein.

Chromosome

A long, organized structure made of DNA wrapped around proteins called histones.

Easy analogy 📖

Think of your genetic material like a book:

Chromosome = book

Gene = chapter

Nucleotides = letters

DNA = the language/code used to write the book

7. Chromosomes

Humans have 46 chromosomes in most body cells.

• You receive 23 chromosomes from your mother.

• You receive 23 chromosomes from your father.

• Together → 46 chromosomes

Where are chromosomes?

Chromosomes are located in the nucleus of most human cells.

8. How DNA Fits Inside the Cell

DNA is extremely long, so it needs to be packaged tightly.

Packaging:

DNA → wraps around histone proteins → forms nucleosomes → becomes more tightly packaged → chromosome

Histones help organize DNA and reduce tangling.

Important:

DNA wraps around histone proteins to help package and organize it.

9. Genes

A gene is a short segment of DNA that contains genetic instructions.

Genes contain information that can help determine traits and provide instructions for making proteins.

Humans have approximately 20,000 genes.

Each chromosome contains many genes.

10. The Genetic Code

The genetic code comes from the order/sequence of nucleotides in DNA.

For example:

A T C G T A C G...

The order of these bases contains information.

Changing the sequence can potentially change the instructions.

Mutation

A mutation is a change in the DNA sequence.

A mutation can involve a change in one or more nucleotides.

11. DNA Codes for Proteins

One of the most important ideas:

DNA contains the instructions for making proteins.

DNA → amino acids → protein

The DNA code determines the order in which amino acids are assembled.

A chain of amino acids forms a protein.

Proteins are important because they:

• Make up many structures in our bodies

  • Muscles

  • Skin

  • Hair

  • Parts of bones

• Include enzymes, which control chemical reactions

• Include many hormones

• Help cells perform many different functions

12. Genes and Proteins

A simplified way to understand it:

Gene
↓
contains DNA instructions
↓
Amino-acid sequence
↓
forms
↓
Protein
↓
performs a function in the body

So when your worksheet says:

"Each gene has the code for putting together one protein"

the basic idea is that genes contain instructions used to produce proteins.

13. Reading the DNA Code

The DNA sequence is read in a particular order.

The bases are organized into groups of three, called codons when referring to the genetic code.

Each codon corresponds to an amino acid (with some codons serving as start/stop signals in the genetic code).

Example from your worksheet:

TAC TGC CTA GTC GGC GTT CGC

The worksheet uses a DNA/amino-acid chart to translate the sequence into amino acids.

The key idea to remember is:

The sequence of DNA bases determines the sequence of amino acids, which determines the protein that is produced.

14. Important Vocabulary

🧠 Super Important Things to Memorize

DNA structure

Nucleotide = sugar + phosphate + base

Four bases

A, T, C, G

Base pairs

A ↔ T

C ↔ G

Hydrogen bonds

A–T = 2

C–G = 3

DNA organization

DNA → nucleosome → chromosome

Genetics

DNA → genes → instructions → proteins

Humans

46 chromosomes

23 from each parent

~20,000 genes

Mutation

Mutation = change in DNA sequence

⭐ The big picture

If you only remember one chain from this lesson, remember:

DNA
↓
contains genes
↓
genes contain instructions
↓
instructions are based on the sequence of nucleotides
↓
instructions determine the order of amino acids
↓
amino acids form proteins
↓
proteins perform many functions and help determine our traits.