Inheritance: Genes and DNA

• Inheritance is the study of how traits are passed from parents to offspring.

Genetic Basics

• The genome is the complete set of genetic material in an organism.
• DNA is a double helix of two strands linked by paired bases.
• Genes are sections of DNA that code for specific proteins and control our characteristics.
• Chromosomes are long DNA strands coiled with histone proteins located in the nucleus.

DNA and RNA

• DNA and RNA are nucleic acids that carry genetic information.
• Base pairs in DNA and RNA include adenine, thymine, guanine, cytosine, and uracil.
• DNA (Deoxyribonucleic Acid) is a double helix polymer containing genetic information; it is found in chromosomes in the cell nucleus.
• DNA comprises deoxyribose sugar, a phosphate group, and a base (adenine, thymine, guanine, or cytosine).
• The sequence of bases in DNA determines the sequence of amino acids in proteins.
• RNA (Ribonucleic acid) is a single-stranded molecule responsible for protein synthesis.
• RNA, which is made of ribose sugar, phosphate, and a base (A, U, G, C), transfers code from DNA in the nucleus to ribosomes in the cytoplasm.

Protein Synthesis

• Protein synthesis occurs in two stages: transcription and translation.
• During transcription, DNA is transcribed into mRNA (messenger RNA).
• The DNA helix unwinds and unzips with the help of the enzyme DNA helicase in the nucleus.
• mRNA matches to its complementary base on the DNA strand and moves out of the nucleus.
• During translation, the ribosome translates mRNA into proteins.
• mRNA attaches to a ribosome that reads its bases in codons (groups of three).
• Start/stop codons in mRNA signal the beginning and end of the code for a protein.
• A codon codes for a specific amino acid, and tRNA has matching anticodons.
• tRNA brings amino acids to the ribosome, where they join when the base pairs on codons and anticodons match.
• Eventually forming a chain of amino acids, and the protein released is folded to form its structure when the stop codon is reached.

Genetic Traits and Inheritance

• The genotype is the combination of alleles that control a characteristic.
• The phenotype is the physical characteristic that can be observed.
• A dominant allele will express its phenotype regardless of whether the genotype is homozygous (when the two alleles of a gene are the same) or heterozygous (when the two alleles of a gene are different); it only needs to be inherited once.
• A recessive allele needs to be inherited from both parents to show up in the phenotype.
• Codominance is the equal expression of equally dominant alleles in the phenotype.
• Polygenic inheritance occurs when phenotypes are controlled by more than one gene, allowing for a wide range of combinations of phenotypes.
• Monohybrid inheritance occurs when phenotypes are controlled by a single gene.

Genetic Diagrams

• A Punnett square shows combinations of alleles in offspring, always showing the dominant allele first.
• A family pedigree traces the pattern of inheritance of characteristics through generations.

Cell Division

• Mitosis is cell division that produces two genetically identical diploid daughter cells for growth, repair, cloning, and asexual reproduction; it undergoes one division.
• During mitosis, the cell grows, and all 46 chromosomes are replicated (96 chromosomes).
• The cell divides, and the chromosomes split, forming the daughter cells.
• Meiosis is cell division that produces four genetically different haploid daughter cells for the formation of gametes to increase genetic variation; it undergoes two divisions.
• In meiosis, the cell grows, and all 46 chromosomes are replicated (96 chromosomes).
• The cell divides into two cells, each with 46 chromosomes, which then divide into four haploid cells, each with 23 chromosomes.

Variation

• Variation refers to differences in DNA sequences among individuals within the same species.
• Gametes are genetically different due to meiosis.
• Fertilization of gametes is random and increases genetic variation.
• Genetic variation means phenotypes depend on inherited alleles from parents.
• Environmental variation means characteristics are caused by differences in lifestyle, climate, or diet.
• Combined, genes decide inherited characteristics, and the environment affects how they develop.

Mutations

• Mutations are rare, random, and spontaneous changes in genes (DNA) that can be inherited and can occur in any chromosome.
• Mutations alter the base pair sequence in DNA, which can code for different proteins, leading to different proteins being made, which affects the phenotype.
• Effects of mutation are most often none or small, but rarely have a significant effect.
• Increasing mutation rates can be caused by ionizing radiation or chemical mutagens that damage bonds and cause changes in base sequences.
• Increased rates of mutation can cause cells to become cancerous.

Natural Selection

• Evolution is the cumulative change in the genetic makeup of a population over many generations, based on the concept of natural selection.
• Variation exists within species, caused by different genes from sexual reproduction and mutations.
• Some phenotypes give individuals a survival and reproductive advantage.
• Individuals with advantageous genes are more likely to pass their genes on at a higher rate, and over generations, the advantageous phenotype becomes the norm.

Antibiotic Resistance

• Antibiotics are chemicals that kill or inhibit the growth and reproduction of bacteria.
• Antibiotic resistance occurs when bacteria become resistant to antibiotics, reproducing and evolving in short time spans.
• Variations caused by mutations occur, which can cause bacteria to be resistant.
• These resistant bacteria do not die when treated with antibiotics, reproduce with less competition, and pass on their genes for antibiotic resistance, leading to a whole population of bacteria becoming antibiotic resistant.
• Causes of antibiotic resistance include unnecessary overuse of antibiotics, such as treating non-serious infections, failing to finish prescribed courses, or routine use in agriculture.
• The effect of antibiotic resistance is that it becomes harder to find working antibiotics, and infections become harder to control, so it is important to always finish a course of antibiotics; otherwise, the worse bacteria will be left and reproduce, and become resistant to that antibiotic.