DNA and Genes

DNA is a double helix whose base pairs A-T and G-C carry genetic information. A gene is a stretch of DNA that codes for a protein: it is transcribed in the nucleus (DNA→mRNA) and translated at the ribosome into protein. Humans have 23 pairs of chromosomes (46 in all), and genes sit on those chromosomes. On this page, change the helix rotation and the expression step to explore DNA structure and gene expression.

DNA Structure

🧬 Intuition — Twisted Ladder

①DNA is a double helix (twisted ladder)

②Rungs = base pairs (A-T, G-C)

③Sides = sugar-phosphate backbone

④The base sequence carries genetic information

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Gene Expression

🔄 DNA to Protein

①Gene: a section of DNA encoding one protein

②Transcription: DNA → mRNA

③Translation: mRNA → amino acids → protein

④Proteins do the actual work in cells

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Chromosomes and Genes

🔬 Hierarchy

①Chromosomes sit inside the cell nucleus

②Chromosome = DNA + protein (histone)

③Genes lie along DNA

④Humans: 23 pairs (46 chromosomes)

⑤About 20,000–25,000 genes

Summary

DNA Structure

DNA = double helix (A-T, G-C pairs)

A pairs with T, G pairs with C via hydrogen bonds

Gene Expression

DNA → (transcription) → mRNA → (translation) → protein

How genetic info becomes a protein

Human Chromosomes

Humans = 23 pairs (46 total)

22 autosomes + 1 sex chromosome pair (XX or XY)

🎯 Exam Points

①DNA: double helix, the genetic material

②Base pairs: A-T, G-C (complementary)

③Gene: a region of DNA coding for a protein

④Transcription (nucleus) → translation (ribosome) = gene expression

⑤Humans: 46 chromosomes (23 pairs), sex chromosomes XX/XY

Examples and Unit Test

Example 1

If one strand of a DNA double helix has the base sequence A-T-G-C, what is the base sequence of the matching opposite strand?

In DNA, bases always pair as A with T and G with C (complementary pairing).

So the strand opposite A-T-G-C is T-A-C-G.

▸ T-A-C-G (A↔T, G↔C complementary pairing)

Because base pairs are complementary, knowing one strand tells you the other.

Example 2

Write in order the steps by which genetic information in DNA becomes a protein, and explain where transcription and translation each take place.

Transcription: inside the nucleus the information in DNA is copied into mRNA.

Translation: the mRNA moves to a ribosome, which links amino acids in order to build a protein.

▸ DNA →(transcription, nucleus) → mRNA →(translation, ribosome) → protein

The base sequence of a gene decides the order of amino acids in a protein.

Unit Test

Which statement about DNA and genes is correct?

①In DNA, A pairs with G

②Translation takes place inside the nucleus

③A gene is a region of DNA that carries the information for a protein

④Humans have 46 pairs of chromosomes

⑤mRNA is a substance that directly breaks down proteins

▸ ③ A gene is a region of DNA that carries the information for a protein

In DNA, A pairs with T (① wrong), and translation takes place at the ribosome while transcription is in the nucleus (② wrong).

Humans have 23 pairs (46 chromosomes) (④ wrong), and mRNA is a messenger that carries information (⑤ wrong); a gene is a region of DNA carrying the information for a protein (③ correct).

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