Molecular Biology of the Gene Chapter 12 - 1 BIOL1000 Dr. Mohamad Termos

Objectives Following this lecture, you should be able to describe: DNA molecules Duplication Mutations Mitosis

DNA Molecules DNA molecules are large polymers made of nucleotides. Each nucleotide is made up of a deoxyribose sugar, a phosphate group, and nitrogenous base. These nucleotides stack up using sugar-phosphate bonds to make one side of the double helix

DNA Molecules DNA nucleotides can have any one of four bases attached to the sugar (deoxyribose). The four possible bases are thymine (T), cytosine (C), adenine (A), and guanine (G). The single-ring bases are called pyrimidines, whereas the larger, double-ring bases are called purines.

DNA Molecules DNA is double stranded. The two strands of DNA are linked together by hydrogen bonds. Adenine always pairs with thymine, and guanine always pairs with cytosine. This is called base pairing. Once base-paired, the two strands coil up like a rope ladder, giving us the familiar double-helix structure of DNA

The DNA Genome The sequence of nucleotides are what make up our genes. DNA contains many different kinds of genes. Each gene contains the info needed to synthesize one kind of protein. A single molecule of DNA has many, many different genes on it. The human genome contains ~100,000 genes. The genome is contained in every single one of your cells

Biology, 9th ed,Sylvia Mader Chapter 13 Structure of DNA Nucleotides: Purine bases (double ring) Adenine (A) Guanine (G) Pyrimidine bases (single ring) Thymine (T) Cytosine (C) DNA Structure & Function

Biology, 9th ed,Sylvia Mader Chapter 13 Chargaff’s Rules The amounts of A, T, G, and C in DNA: Varies between individuals of a species Varies more from species to species There are equal amounts of: A & T G & C Human chromosome estimated to contain, on average, 140 million base pairs DNA Structure & Function

Complementary base pairing Biology, 9th ed,Sylvia Mader Chapter 13 Complementary base pairing Purine is always bonded to a pyrimidine: A bonded to T, G bonded to C DNA Structure & Function

Biology, 9th ed,Sylvia Mader Chapter 13 Watson and Crick Model Watson and Crick, 1953 Constructed a model of DNA Double-helix model - similar to a twisted ladder Sugar-phosphate backbones make up the sides - Bases make up the rungs and are bonded by hydrogen bonds DNA Structure & Function

Biology, 9th ed,Sylvia Mader Chapter 13 DNA Replication Process of copying a DNA molecule to result in two DNA molecules -Semi-conservative - a strand of the original DNA is present in each resulting copy of DNA -replication is necessary before a cell can divide DNA Structure & Function

Biology, 9th ed,Sylvia Mader Chapter 13 DNA Replication Steps in replication: 1- Unwinding, 2- Complementary Base Pairing, 3- Joining DNA Structure & Function

Semiconservative Replication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5′ 3′ G C G C G A T A region of parental DNA double helix T A C G DNA polymerase enzyme A T A T G G T C G C G C G C A A C A T C G C region of replication: new nucleotides are pairing with those of parental strands T G A A T T A T T A G C T C A T A G A T A T A A G A G G T region of completed replication T A C C C A G C 3′ G n e w o l d A s t r a n d s t r a n d 5′ daughter DNA double helix old strand new strand daughter DNA double helix

Mutations You might also be wondering: what happens if something goes wrong? What if the wrong RNA base pairs with the DNA during transcription? Well, those “wrongs” are called mutations. Mutations are errors in the genetic code (DNA) that can cause errors in the final amino acid sequence. Sometimes, these errors, even just a change in one amino acid, can cause huge problems. For example, sickle-cell anemia is a disease caused by a change in one amino acid.

Mutations Mutation can cause change in just one amino acid (like base substitution in sickle-cell anemia) or many amino acids, in the case of a base deletion. Sometimes, it can create a prematurely short protein, if a codon that normally codes for an amino acid gets changed into a stop codon instead. However, mutations can be a good thing. What if the change in protein structure helped the protein work better? We might want to keep this change! If we do, then this is one way we are able to select for better survival factors. These kinds of mutations work towards evolutionary change.