DNA Structure and Replication Chapter 5 Nucleotides Genes

DNA Structure and Replication – Chapter 5 Nucleotides, Genes, Errors, Recombinant DNA

Cytology and Genetics Why do you think these two ideas are connected? Aristotle heredity was related to the power of male semen Bloodline hereditary traits were located in the blood Nucleus (1831) helped to clear up some of the confusion Microscope allowed Fleming (1882) ~ separation of “threads” Van Benden (1882) ~ roundworms had 4 chromosomes, sperm and egg of roundworm only had 2 chromosomes Weisman (1887) ~ meiosis

Chromosomes and Heredity Mendel’s laws were created without an understanding of meiosis or an understanding of chromosomes. His findings went largely unnoticed until the early 1900’s. Sutton and Boveri (1902) were the two who realized that the behaviour of chromosomes during meiosis was related to the behaviour of traits in Mendel’s experiments.

Chromosomes and Heredity The combined research of the three formed the chromosome theory of inheritance Genes are located on chromosomes and chromosomes provide the basis for segregation and independent assortment of genes.

Morgan’s Discovery Thomas Morgan (1910) crossed two red eyed flies and produced a white eyed offspring. Thought it was related to the law of dominance in a monohybrid cross. However, when the white eyed male was crossed with a red eyed female, he could not produce a female with white eyes. The coding for eye colour was determined to be on the X chromosome. p. 166, fig 3

Morgan’s Discovery

Sex-linked Inheritance Some traits that are passed on depend on the sex of the parent carrying the trait. Transmission of genes that are located on one of the sex chromosomes, X or Y Gene that is located on X chromosome is called X-linked and Y is known as Y-linked. X-linked is more common then Y linked as the X chromosomes is larger. Examples Y of X-linked are: colour blindness, hemophilia. linked disorders are rare, and almost always cause infertility

Chromosomes and Gene Expression Females have two copies of the X chromosome however the amount of proteins in males and females is equal. What happens is that one of the X chromosomes is inactivated randomly, it is referred to as a Barr body. It is important to remember that different X chromosomes are inactivated in different cells. Tortoiseshell coat colour in cats is an example (black and orange).

Chromosomes and Gene Expression

Structure of DNA = deoxyribonucleic acid DNA gets copied during mitosis and each new cell gets a complete copy Chromosomes vs. Proteins Which one carried genetic information? This was not figured out until the 1950’s - Alfred Hershey and Martha Chase – 1952. - They completed an experiment with bacteriophages - - Viruses that attack bacteria and use the bacteria’s structures quickly reproduce new viruses in the cell to - Outer coating made of protein and inside carries DNA - 35 S - DNA went inside the cell, therein this is what was being transmitted therein it must be the genetic information. on protein and 32 P on DNA

Nucleotides P. A. Levine – work done earlier then Hershey. Chase • He determined that DNA is made up of nucleotides, which are themselves made of three parts: 1. Sugar – deoxyribose 2. Phosphate group (phosphoric acid) 3. Nitrogenous base – • adenine, guanine = purines • Thymine, cytosine = pyrimidine Chargaff = ratios A=T C=G

Double Helix The Race Was On! Certain things were known at this point: DNA is made of nucleotides Nucleotides are linked together Ratios Compression (1 m if stretched out) Rosalind Franklin – X ray diffraction DNA was a giant spring or coil (2 nm) Watson and Crick put all the information together Double Helix Complementary Base Pairs

Nucleotide Sequences Only 4 different nitrogenous bases Similar organisms have similar DNA Remember a computer uses only 2 pieces of information – electrical impulse or no Genes and Proteins 20 different AA that are linked together to create polypeptides Sequence of AA is determined by the sequence of nucleotides in the DNA.

Nucleotide Sequences Genetic Code If each nucleotide coded for one amino acid, we would only need 4 amino acids If two nucleotides coded for one amino acid, we still would not have enough combinations. So we have three nitrogenous bases to code for one amino acid (although there are now 64 different combinations). However, in some cases two or more codons code for the same amino acid Stop codons = indicate that no more amino acids should be added Start codon (AUG) – codes for methionine and it is a start codon

Nucleotide Sequences

Transposons (Jumping Genes) Specific segments of DNA that can move along the chromosome. Flint or Indian Corn The individual grains are purple with white streaks or mottling. This mottling effect defies Mendel's basic principles of genetics because individual grains may be multicolored rather than a single color. In the pigmented layer of corn grains, the position of transposons may inhibit or block pigment production in some cells for each kernel. For example, if the transposon moves to a position adjacent to a pigment-producing gene, the cells are unable to produce the purple pigment. The reddish-purple patterns caused by transposons may be blotches, dots, irregular lines and streaks.

Transposons The different cards represent a linear sequence of genes on a chromosome. The ace of spades represents a transposon that moves to different positions on the chromosome. The two of spades represents the gene for purple pigmentation in developing corn kernel cell. When the transposon (ace of spades) moves to a position adjacent to the gene for pigmentation (jack of diamonds), the pigmentation gene is blocked and no purple is synthesized (white area) 1940’s recognized in 1983…why?

Recombinant DNA • Bacteria often provide the appropriate machinery (enzymes and ribosomes) for us to produce proteins from a specific gene insulin • Bacteria have small circular pieces of DNA called plasmids within their cytoplasm

Genetic Research & Tech Human Genome Project – to locate all of the believed 80 000 – 100 000 genes on our 46 chromosomes international flavour Actually 25 000 -30 000 The human genome contains over 3 billion chemical nucleotide bases (A, C, T, and G). The average gene consists of 3000 bases, but sizes vary greatly, with the largest known human gene being dystrophin at 2. 4 million bases. With this knowledge if a genetic disorder is known, drugs could be used to treat the disorder instead of just the symptoms or gene therapy could be utilized What about the problems associated with this?

DNA Fingerprinting Isolate DNA from biological materials from the crime scene (hair or skin). Compared to that of the suspect Try p. 183 = Reading a DNA Fingerprint. Can also be used to determine relatedness Pedigrees Conservation Important Biologists to managing populations of endangered or threatened species

Errors in Replication Mistakes There are few and far between are special enzymes that read the DNA for errors. Mutations Useful can be: = positive mutation Harmful = negative mutation Inconsequential Mutagens = neutral mutation cause mutations to occur at a higher frequency.

Gene Therapy Treatment vs. Cure Insertion of new (working) gene to replace one that is not working Cell specific Issues This with this therapy? is the area of hope