Genetics & the DNA: The Science of Heredity

The Genetic Code DNA is often called the blueprint of life. DNA-Deoxyribonucleic acid, a self-replicating material present in nearly all living organisms as the main component of chromosomes. It is the carrier of genetic information. In simple terms, DNA contains the instructions for making proteins within the cell. Why is DNA called the blueprint of life?

Chromosomes and DNA Chromosomes are found in the cell’s nucleus and are made up of genes. Genes are sections of chromosomes made up of DNA. {Ask students where the chromosomes are in this picture. Or ask them where the DNA is. Remind them that the mitochondria also have DNA.}

The Shape of the Molecule/The Double Helix Molecule DNA is a very long molecule. The basic shape is like a twisted ladder or zipper. This is called a double helix. The DNA double helix has two strands twisted together. {Show students a model of the double helix. Explain what a spiral is and a helix is.}

One Strand of DNA The DNA molecule is made of units called nucleotides. Each nucleotide is made of a phosphate, a sugar called deoxyribose, and a nitrogen base. {Point to the 3-D mode, if you have one, to show the parts as you discuss them.}

DNA has four different Nitrogen bases: Cytosine C Thymine T Adenine A Guanine G These four bases are abbreviated by using their respective first letters.

Hydrogen Bonds When making hydrogen bonds, cytosine always pairs up with guanine, And adenine always pairs up with thymine.

Copying DNA:DNA REPLICATION Step 1- DNA unwinds and unzips Step 2- Once the molecule is separated it copies itself. The new strand of DNA has bases identical to the original.

DNA by the numbers Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. If you unravel all the DNA in the chromosomes of one of your cells, it would stretch out 2 meters. If you did this to the DNA in all your cells, it would stretch from here to sun more than 400 hundred times!

History of Genetics Genetics is the study of heredity. Heredity is the passing of physical characteristics, or traits, from parents to offspring (from generation to generation). Chromosomes are made up of genes, which are made up of DNA. Genetic material (DNA, genes, chromosomes) is found inside the nucleus of a cell. Fertilization is the process in sexual reproduction in which an egg cell and a sperm cell join to form a new organism. Gregor Mendel is considered “The Father of Genetics“.

Gregor Mendel For seven years Mendel experimented with pea plants. Mendel was one of the first to apply an experimental approach to the question of inheritance. He used pea plants because: They were available They reproduced quickly They showed obvious differences in the traits He understood that there was something that carried traits from one generation to the next- “FACTOR”. In the mid-1800s, the rules underlying patterns of inheritance were uncovered in a series of experiments performed by an Austrian monk named Gregor Mendel.

Mendel developed the “The particulate hypothesis” which is the idea that parents pass on discrete heritable units (genes). To test the particulate hypothesis, Mendel crossed purebred plants that had two distinct and contrasting traits—for example, purple or white flowers. A purebred organism is the offspring of many generations that show the same form (same allele) of a trait.

Mendel cross-fertilized purebred plants that were different in just one trait—in this case, flower color. He then allowed the hybrids (the F1 generation) to self-fertilize. Hybrid plants: offspring resulting from crossing two purebred plants. It has two different alleles for the same trait(one allele for purple color and one allele for white color).

Mendel Particulate Experiment P generation (parental generation) F1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring. Allowing these F1 hybrids to self-pollinate produces: F2 generation (second filial generation). It is the analysis of this that lead to an understanding of genetic crosses.

Mendel studied seven TRAITS in the garden pea

Chromosomes Homologous chromosomes: one of a matching pair of chromosomes, one inherited from each parent.

Alleles: alternative versions of a gene. The gene for a particular inherited character (trait) resides at a specific locus (position) on homologous chromosome. For each character (trait), an organism inherits two alleles, one from each parent. Scientist use letters to represent alleles. A dominant allele is symbolized with a capital letter. A recessive allele is symbolized by the lowercase version of the same letter.

How do alleles differ? Alleles (P, p) Homozygous dominant-(PP) Heterozygous-( Pp) Homozygous recessive (pp) P--Allele for purple flower p-- Allele for white flower Dominant allele - a term applied to the trait (allele) that is expressed regardless of the second allele. Recessive allele- a term applied to a trait that is only expressed when the second allele is the same (example: pp White flowers). Genotype- the types of genes (alleles) present. Example PP. Phenotype- what it looks like, physical appearance of the trait. Example Purple flower.

Probability and Punnett Squares Punnett square: chart that shows all the possible ways alleles can combine in a genetic cross & the probabilities of possible outcomes in a genetic cross. Probability- the chances/ percentages that something will occur. Pedigree- is a chart or “family tree” that shows the presence or absence of a trait according to the relationships within a family across several generations. It can be used to diagnose genetic disorders.

Probability and Punnett Squares

Genotype versus Phenotype. How does a genotype ratio differ from the phenotype ratio?

gPunnett Square Problems What is the genotypic ratio of the offspring in this genetic cross? What is the probability that the offspring in this genetic cross will be short? What are the genotypes of the offspring? What are the phenotypes of the offspring? What is the genotype of the offspring? What is the phenotype of the offspring?

Mendelian Genetics Alleles- the different forms of a gene. Dominant allele- traits that are expressed, always shows up in the organism when the allele is present. Recessive allele- traits that are covered up, is hidden whenever the dominant allele is present. Punnett Squares- chart that shows all the possible ways alleles can combine in a genetic cross. Probability- the chances/ percentages that something will occur. Genotype- the types of genes (Alleles) present. Phenotype- what it looks like, physical appearance of the trait. Homozygous- two of the same alleles. Heterozygous- two different alleles.

CELL DIVISION & MEIOSIS There are two types of cells: somatic cell (body cells) and sex cells or gametes. These cells divide through two different processes: Body cells through mitosis. Sex cells through meiosis. Body cells are diploid: two alleles of each gene. Sex cells are haploid: one allele of each gene.

MEIOSIS Is the process by which the number of chromosomes is reduced by half as sex cells form. Meiosis produces 4 haploid sex cells. Humans have 46 chromosomes in their body cells. Each human sex cell has 23 chromosomes.

Asexual vs. Sexual Reproduction Fertilization: process by which an egg and a sperm join to produce a new organism. Sexual reproduction requires meiosis. Asexual reproduction requires mitosis. ASEXUAL vs. SEXUAL REPRODUCTION