Traits such as a face that looks like a parent's, blood type and eye color start from information written in the DNA inside the cell nucleus. DNA is a double helix, like a twisted ladder, and its rungs are base pairs, A with T and G with C.
A stretch of DNA that holds the information for making a particular protein is a gene, and that information becomes a protein through transcription and translation.
Rotating the first diagram shows that A always faces T and G always faces C; stepping through the second shows the information passing from DNA to mRNA and then into a protein.
A human body cell has 46 chromosomes (23 pairs), and about 20,000 genes are strung along the DNA inside them.
Structure of DNA
🧬 A Twisted Ladder
①DNA is a double helix: two strands twisted around each other
②The sides of the ladder are a backbone of alternating sugar and phosphate
③The rungs are base pairs: A (adenine) pairs only with T (thymine), and G (guanine) only with C (cytosine)
④The order of the four bases (the base sequence) is the genetic information
seegongsik.com
The DNA double helix with A-T and G-C base pairs, viewed while rotating
0°
Gene Expression
🔄 From DNA to Protein
①A gene is the part of DNA holding the information for a particular protein
②Transcription: in the nucleus, the gene's base sequence is copied into mRNA
③Translation: the mRNA goes to a ribosome in the cytoplasm, where amino acids are linked in its order to build a protein
④The proteins made act as enzymes or body structures and produce traits
seegongsik.com
mRNA being made from DNA by transcription and turned into a protein by translation
DNA
Chromosomes and Genes
🔬 Nucleus, Chromosome, DNA, Gene
①Chromosomes are found in the cell nucleus
②A chromosome is DNA wound around proteins and packed tightly
③Different stretches of the DNA are each a gene
④A human body cell has 46 chromosomes (23 pairs)
⑤Humans have about 20,000 to 25,000 genes
Summary
DNA structure
DNA = double helix (base pairs A-T, G-C)
Adenine pairs with thymine and guanine with cytosine through hydrogen bonds
Gene expression
DNA → (transcription) → mRNA → (translation) → protein
A gene's base sequence sets the order of amino acids in the protein
Human chromosomes
Humans = 46 chromosomes (23 pairs)
22 pairs of autosomes + 1 pair of sex chromosomes (female XX, male XY)
🎯 Common Test Points
①DNA is the genetic material, with a double-helix structure
②Bases pair A-T and G-C (complementary pairing)
③A gene is the part of DNA holding the information for a protein
④Transcription happens in the nucleus, translation at the ribosome
⑤Humans have 46 chromosomes (23 pairs); the sex chromosomes are XX in females and XY in males
Examples and Unit Review
Example 1
If one DNA strand has the base sequence A-T-G-C, what is the sequence of the strand paired with it?
1
In DNA, A always pairs with T and G always pairs with C (complementary pairing).
2
So the strand paired with A-T-G-C is T-A-C-G.
▸ T-A-C-G
Because one strand determines the other, a cell can copy its DNA exactly when it divides.
Example 2
List in order how genetic information in DNA becomes a protein, and say where transcription and translation each take place.
1
Transcription: in the nucleus, the gene's base sequence is copied into mRNA.
2
Translation: the mRNA goes to a ribosome, where amino acids are linked in its order to form a protein.
▸ DNA → (transcription, nucleus) → mRNA → (translation, ribosome) → protein
Changing a single base can change the order of amino acids, which may alter the protein's shape and function.
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
1
In DNA, A pairs with T (① wrong), and translation takes place at the ribosome while transcription is in the nucleus (② wrong).
2
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).