DNA, Genes and Cell Division

• DNA's structure was described by the Watson-Crick model
• DNA stands for deoxyribonucleic acid
• DNA is comprised of nucleotides
• Nucleotides feature a five-carbon sugar, a nitrogenous base, and a phosphate group
• The sugar in DNA is deoxyribose
• Deoxyribose forms the backbone
• Complementary base pairs include Adenine to Thymine, and Cytosine to Guanine
• The DNA molecule is shaped as a double helix

Genes, Chromosomes, and Alleles

• Genes are DNA sections instructing how to make proteins, controlling body functions and traits
• Chromosomes are long DNA strands in the cell nucleus. Humans have 46, organized as 23 pairs; chromosomes carry all genes
• Alleles are gene variants; for eye color, there could be a blue or brown allele. One allele is inherited from each parent

Mitosis and Meiosis

• Mitosis is for growth and repair, producing identical cells
• Meiosis is for sexual reproduction, creating gametes like sperm and egg

Mitosis Stages

• Prophase involves visible chromosomes, nuclear membrane breakdown, and spindle fibers forming
• Metaphase involves chromosomes lining up in the cell's middle and spindle fibers attaching to centromeres
• Anaphase involves sister chromatids separating and moving to opposite sides
• Telophase involves two new nuclei forming around chromosomes
• Cytokinesis involves the cell splitting into two identical daughter cells

Meiosis Stages

• Meiosis involves two divisions: Meiosis I and Meiosis II
• During Prophase I, chromosomes become visible and homologous pairs link up. Crossing over occurs [genetic material exchange]
• Metaphase I involves homologous pairs aligning in the cell's middle
• Anaphase I involves homologous chromosomes pulled to opposite sides [sister chromatids remain together]
• Telophase I & Cytokinesis involves 2 new cells forming, with half the original chromosome number
• Meiosis II separates sister chromatids [like mitosis]
• Prophase II involves new spindle fibers forming in each cell
• Metaphase II involves chromosomes lining up again in the middle
• Anaphase II involves sister chromatids being pulled apart
• Telophase II & Cytokinesis involves cells splitting again, resulting in 4 non-identical daughter cells [gametes], with half the chromosomes

Genetics, Traits, Alleles, Genotype, and Phenotype

• Genetics is the study of traits passed from parents to offspring through genes
• Traits are characteristics of organisms like hair colour, height, or blood type
• Alleles are different forms of a gene; a flower colour gene might have red and white alleles
• Genotype is the allele combination, like the genetic code [BB, Bb, or bb]
• Phenotype is a genotype's physical expression, such as blue eyes or curly hair

Dominant and Recessive Alleles

• Dominant alleles show in the phenotype when present and are represented by a capital letter, e.g., B
• Recessive alleles only show if both alleles are recessive and are represented by a lowercase letter, e.g., b
• BB or Bb shows a dominant trait
• bb shows a recessive trait

Chromosomes

• Homologous chromosomes form matching pairs from each parent, e.g., XX or Xx
• Homozygous genotype is when alleles are the same: BB [homozygous dominant] or bb [homozygous recessive]
• Heterozygous genotype means alleles are different: Bb

Chromosomes and Sex Determination

• Humans have 23 chromosome pairs
• The 23rd pair is the sex chromosomes: XX is female, XY is male
• Males determine the baby's sex through X or Y chromosome contribution, females contribute an X only

Gene and Chromosome Manipulation

• Mutations are changes in DNA that affect genes
• Biotechnology allows humans to alter genes/chromosomes [genetic engineering, cloning, gene therapy]
• Mutations can occur during mitosis or meiosis when cells divide
• DNA copying mistakes can cause changes that are harmful, helpful, or have no effect

Biotechnology Types and Descriptions

• Genetic engineering involves directly changing DNA in an organism [GMOs]
• Cloning involves making a genetically identical copy of an organism
• Gene therapy involves inserting healthy genes to fix disorders
• CRISPR is a tool for precise editing of genes
• Selective breeding involves breeding parents with certain traits together

DNA Structure

• DNA [deoxyribonucleic acid] is a twisted ladder, a double helix
• The sides consist of sugar and phosphate
• The rungs consist of base pairs: Adenine with Thymine, Cytosine with Guanine

Mitosis vs. Meiosis

Variation during Meiosis and Fertilization

• Crossing over occurs in Prophase I where chromosomes swap sections
• Independent assortment occurs in Metaphase I where chromosomes align randomly
• Fertilisation involves random sperm meeting random egg, creating new genetic combinations

Punnett Squares, Genotype, and Phenotype

• Punnett squares are used to dtermine genotype and phenotype
• Example: Brown eyes [B] are dominant, blue eyes [b] are recessive

Pedigrees and Disease Inheritance

• Circles represent females; Squares represent males
• Shaded shapes means the person has the trait/disease
• Skipping generations indicates a likely recessive trait
• Appearing in every generation indicates a likely dominant trait

Mutations and Mutagens

• Mutations are changes in the DNA sequence
• Mutagens cause mutations [UV radiation, chemicals, X-rays]
• Mutations can be beneficial [rarely], neutral, or harmful [cause diseases]

Diseases from Mutation

• Cancer can occur if mutations arise during cell division [mitosis] and damage growth genes
• This causes cells to divide uncontrollably, forming tumors

Biotechnology Functions

• CRISPR acts like "molecular scissors" to edit genes
• Gene Therapy replaces faulty genes with healthy ones
• Genetic Engineering inserts genes into organisms [pest-resistant crops]
• Cloning creates genetically identical copies [Dolly the sheep]

Structure of the Periodic Table

• Elements are ordered by increasing atomic number, equaling the number of protons in an atom's nucleus
• This order reveals recurring properties in periods
• There are 7 periods [rows] in the periodic table
• Each period sees an electron shell being filled
• Atomic number increases moving across a period
• Elements shift from metals to non-metals
• Reactivity and other properties predictably change across a period

Groups

• There are 18 groups, also called families
• Elements in the same group have the same number of valence electrons and have similar chemical properties
• Group 1 elements have one valence electron, making them reactive

Isotopes

• Isotopes are atoms of an element with the same number of protons but a different number of neutrons
• They share the same atomic number but have differing mass numbers
• Many elements have natural isotopes
• Carbon has stable isotopes, Carbon-12 and Carbon-13
• Radioactive isotopes like Carbon-14 are used in radiocarbon dating

Family Groupings in the Periodic Table

Periodic Table Properties

Across a Period [Left to Right]

• Atomic radius decreases
• Ionisation energy increases
• Electronegativity increases

Down a Group [Top to Bottom]

• Atomic radius increases
• Ionisation energy decreases
• Electronegativity decreases

Element Property Comparisons

Group Trends

• Elements in the same group share similar chemical properties from the same valence electrons
• All Group 1 elements form +1 ions

Period Trends

• Properties progressively change across a period

Family Group Contrasts

Alkali Metals vs. Noble Gases

• Alkali metals react strongly, especially with water, and exist only in compound form
• Noble gases are very unreactive because of full valence electron shells

Halogens vs. Alkaline Earth Metals

• Halogens are reactive non-metals that gain electrons
• Alkaline earth metals are reactive metals that lose electrons

Group and Period Significance

Groups

• The group number indicates the number of valence electrons
• Valence electrons control reactivity and bonding

Periods

• The period number indicates the number of electron shells
• Down a group, each element gains an electron shell