3. Structure of DNA The structure of DNA was determined by James Watson and Francis Crick in the early 1950s. DNA is a polynucleotide ; nucleotides are composed of a phosphate, a sugar, and a nitrogen-containing base.

5. A nucleotide

6. Watson and Crick showed that DNA is a double helix in which A is paired with T G is paired with C This is called complementary base pairing because a purine is always paired with a pyrimidine .

8. When the DNA double helix unwinds, it resembles a ladder. The sides of the ladder are the sugar-phosphate backbones, and the rungs of the ladder are the complementary paired bases. The two DNA strands are anti-parallel – they run in opposite directions.

9. DNA double helix

10. Replication of DNA DNA replication occurs during chromosome duplication. An exact copy of the DNA is produced with the aid of DNA polymerase . Hydrogen bonds between bases break and enzymes “unzip” the molecule.

12. DNA replication

13. Ladder configuration and DNA replication

14. Gene Expression A gene is a segment of DNA that specifies the amino acid sequence of a protein. Gene expression occurs when gene activity leads to a protein product in the cell. A gene does not directly control protein synthesis; instead, it passes its genetic information on to RNA, which is more directly involved in protein synthesis.

15. RNA ( ribonucleic acid ) is a single-stranded nucleic acid in which A pairs with U ( uracil ) G pairs with C.

17. Structure of RNA

18. Two processes are involved in the synthesis of proteins in the cell: Transcription makes an RNA molecule complementary to a portion of DNA. Translation occurs when the sequence of bases of mRNA directs the sequence of amino acids in a polypeptide.

19. The Genetic Code DNA specifies the synthesis of proteins because it contains a triplet code : every three bases stand for one amino acid. Each three-letter unit of an mRNA molecule is called a codon .

22. Central Concept or " The Central Dogma" The central concept of genetics involves the DNA-to-protein sequence involving transcription and translation. DNA has a sequence of bases that is transcribed into a sequence of bases in mRNA. Every three bases is a codon that stands for a particular amino acid.

23. Overview of gene expression

25. Transcription and mRNA synthesis

26. Translation Translation is the second step by which gene expression leads to protein synthesis. During translation, the sequence of codons in mRNA specifies the order of amino acids in a protein. Translation requires several enzymes and two other types of RNA: transfer RNA and ribosomal RNA .

27. Review of Gene Expression DNA in the nucleus contains a triplet code; each group of three bases stands for one amino acid. During transcription, an mRNA copy of the DNA template is made.