Genetics

Outline Structure of DNA DNA Functions Transcription Translation Mutation Cytogenetics Mendelian Genetics Quantitative Traits Linkage

Structure of DNA DNA molecule organized into chain of nucleotides composed of three parts: Nitrogenous Base 5-Carbon Sugar (Deoxyribose) Phosphate Group Nucleotides bonded to each other forming a helix. Hydrogen bonds hold each base on one side of the helix to another base on the other side.

Structure of DNA Four types of DNA nucleotides: Purines Adenine (A) and Guanine (G) Pyrimidines Cytosine (C) and Thymine (T)

Fig. 13.3

DNA Functions Storage of Genetic Information Gene - Segment of DNA that directs protein synthesis. Protein may act as an enzyme influencing cell activities. Genome - Sum total of DNA in an organism’s chromosomes. Codon - Groups of three nucleotides.

DNA Functions Replication of Information Occurs during S phase of cell cycle. Strands of double helix unzip allowing DNA polymerase to pair individual nucleotides with the template strands. Semi-Conservative Replication Occurs simultaneously in both directions, and begins at several points simultaneously.

DNA Replication

DNA Functions Expression of Information Different subset of the genetic information is read in each cell type. Expression requires two processes. Transcription - Copy of gene message using RNA. Translation Messenger RNA (mRNA) translated to produce proteins.

Transcription Three different types of RNA produced: Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA) RNA Synthesis Nucleotides added to single stranded DNA molecule by RNA polymerase. Only portions of the genome replicated. Remainder is noncoding DNA.

Transcription Promoter Region signals transcription enzymes to attach to the DNA. Single-stranded RNA transcript produced. Chromosomes contain genes for building tRNA. Each form of tRNA has a specific anticodon loop. Base pairs with codon. Genes for rRNA also transcribed in the nucleus - Used to construct ribosomes.

Translation mRNA transcripts are decoded into proteins. Genetic code based on codons. 64 possible combinations Code for 20 amino acids. tRNA acts as a translator. Anticodon binds to mRNA codon. Start of translation signaled by a ribosome in the cytoplasm binding to the mRNA.

Fig. 13.5

Fig. 13.6

Fig. 13.6a

Fig. 13.6b-1

Fig. 13.6b-2

Fig. 13.6b-3

Fig. 13.6b-4

Fig. 13.6c

Central Dogma of Molecular Dogma

Mutation Mutations - Changes in DNA sequence. Mutagens - Agents that alter DNA. Ultraviolet light Ionizing radiation Somatic Mutation - Occurs in body cell. Germ-Line Mutation - Occurs in tissues that will produce sex cells. Passed on to future generations. All genetic variability due to mutations.

Cytogenetics Cytogenetics is the study of chromosome behavior and structure, from a genetic point of view. Changes in Chromosome Structure Inversion - Chromosomal piece is broken and reinserted in the opposite orientation. Translocation - Chromosomal piece breaks off and attaches to another chromosome. Important in speciation.

Cytogenetics Changes in Chromosome Number Mistakes during chromosome pairing and separation can result in gametes carrying extra or missing chromosomes. Aneuploid - Carries, or is missing, an extra chromosome(s). Polyploid - Has at least one complete extra set of chromosomes.

Mendelian Genetics Mendel crossed self-pollinated pea plants and recorded his observations. Crossed a tall plant with a short plant. Parental Generation (P) All offspring were tall. First Filial Generation (F1) Cross offspring yielded a ratio of three tall individuals to one short individual. Second Filial Generation (F2)

Mendelian Genetics Law of Unit Characters Factors (Alleles), which always occur in pairs, control the inheritance of various characteristics. Genes are always at the same position (locus) on homologous chromosomes. Law of Dominance For any given pair of alleles, one (dominant) may mask the expression of the other (recessive).

Mendelian Genetics Phenotype refers to an organism’s physical appearance. Genotype refers to the genetic information responsible for contributing to the phenotype. Homozygous - Both alleles identical. Heterozygous - Alleles are contrasting.

Monohybrid Cross A cross is made between two true-breeding parents differing for a single trait, producing an F1 generation. These plants are intercrossed to produce an F2 generation.

Fig. 13.10

Fig. 13.11

Dihybrid Cross Cross made for parents differing in two traits. Mendel’s Law of Independent Assortment Factors (genes) controlling two or more traits segregate independently of each other. Linked genes do not segregate independently. Punnett Square used to determine genotypes of the zygotes.

Fig. 13.12

Mendelian Genetics Backcross A cross between a hybrid and one of its parents. Can be used to test inheritance theory. Incomplete Dominance Some genes exhibit traits in which a heterozygote is intermediate in phenotype to the two homozygotes.

Fig. 13.13

Quantitative Traits Quantitative traits exhibit a range of phenotypes rather than discrete phenotypes studied by Mendel. Molecular geneticists are able to identify chromosomal fragments, quantitative trait loci, associated with quantitative traits.

Extranuclear DNA Entranuclear DNA is found in both mitochondria and chloroplasts. Endosymbiont Hypothesis theorized mitochondria and chloroplasts were free-living, and at some point became incorporated into cells of organisms that evolved into plants and established a symbiotic relationship.

Linkage and Mapping Genes that are together on a chromosome are said to be linked. Each gene has a specific location (locus) on a chromosome. Crossing-over should be more likely between two genes located far apart on a chromosome that between two genes located closer together. Crossing over frequency can be used to construct a genetic map. 1 map unit = 1% crossing over.

Hardy-Weinberg Law Hardy-Weinberg states the proportions of dominant alleles to recessive alleles in a normally interbreeding population will remain the same from generation to generation in the absence of external forces.

Review Structure of DNA DNA Functions Transcription Translation Mutation Cytogenetics Mendelian Genetics Quantitative Traits Linkage

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