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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