Year 10 Biology & Chemistry

DNA

• Deoxyribonucleic Acid

• A molecule that is the blueprint of all living things. 

• Instructions needed for an organism to develop, survive and reproduce.

Structure of DNA

• 2 backbones forming a double helix.

• ‘Rungs’ made up of bases: Adenine (A), thymine (T), guanine (G) and cytosine (C).

  

DNA Molecule Structure

• A DNA nucleotide (monomer) has 3 parts:

  • A phosphate group

  • A deoxyribose sugar

  • A nitrogenous base (one of A, T, C or G)

  

DNA Base Pairing

• Base pairs are complementary.

• Adenine (A) and Thymine (T) join together: A–T

• Guanine (G) and Cytosine (C) join together: G–C

RNA

• Ribonucleic Acid

• Can act as a messenger (mRNA).

• mRNA is an RNA copy of our DNA, and is used to send information on protein making from the nucleus to the ribosomes.

• Bases: adenine (A), uracil (U), guanine (G) and cytosine (C).

• A pair with U instead of T.

DNA and RNA Differences

DNA	RNA
Double Stranded	Single stranded
Deoxyribose sugar	Ribose sugar
A,T,C,G Bases	A,U,C,G Bases
Longer	Shorter

Chromosomes

• Histone proteins organize DNA by wrapping it around themselves into chromosomes.

• Chromosomes carry DNA in units called genes..

• Humans have 46 (23 homologous pairs).

• Chromosomes are inherited in pairs, one from each parent.

• 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males).

  

Genes

• The genome is an organism's complete set of genes.

• Sections of DNA molecules that carry instructions for building specific proteins.

• Can be inherited, passing down traits.

• The order of nucleotide bases within a gene (genetic code), dictates the protein it will produce, and the trait it influences.

The Genetic Coding Process

• DNA gets copied onto messenger RNA.

• mRNA travels out of the nucleus to the ribosomes.

• Bases on the DNA and mRNA are read in triplets.

• Each triplet/codon codes for a different amino acid.

• Amino acids join to form long chains that make proteins.

DNA Replication

• Occurs so there is enough DNA for cell division. Ensures that each new cell gets a copy of the genome.

• Semi-conservative, with each strand of the double helix serving as a template for a new complementary strand.

• The hydrogen bonds DNA double helix unwinds, and each strand acts as a template for the synthesis of a complementary strand.

• Complementary base pairs are added to each template strand during replication.

• Replication results in two identical DNA double helices, each with one old and one new strand.

Mitosis

• Division of somatic (not sex) cells; autosomes.

• Prophase, Metaphase, Anaphase, Telophase: PMAT

Interphase

• Cells spend most of their life in this stage.

• DNA replicates, so each daughter cell has the same amount of DNA as the parent cell.

Prophase

• Chromatids condense into chromosomes and the nucleus starts to break down.

• Centrioles move to opposite ends of the cell, and spindles form from microtubules.

Metaphase

• Chromosomes line up along the middle of the cell.

• Centromeres are attached to a spindle fiber.

Anaphase

• Chromosomes separate and are pulled away from each other by spindle fibres.

Telophase

• Two nuclei form, each with its own set of identical DNA, within two new daughter cells (cytokinesis).

• Chromosomes decondense, and chromatids unwind.

Meiosis

• Produces sex cells, through fertilisation of gametes.

• Two cycles of cell division (meiosis I and II), resulting in four daughter cells. 

• Passes on genetic information from parents to children during sexual reproduction.

Meiosis Extended

• Meiosis produces haploid cells (23 individual) to form gametes for sexual reproduction, ensuring correct chromosome numbers in offspring. Chromosomes are halved from diploid (46) to haploid (23).

• Genetic variation arises from independent assortment and crossing over (genetic exchange between homologous chromosomes).

• Nondisjunction is where chromosomes fail to separate properly leading to an abnormal number of chromosomes. It can lead to trisomies like Down syndrome, affecting genetic health.

Mitosis and Meiosis Differences

Mitosis	Meiosis
Genetically identical	Genetically diverse
Diploid (23 pairs, 46 individual)	Haploid (23 individual)
Growth and Repair	Sexual Reproduction
Longer	Shorter

Number of Chromosomes

	Number of chromosomes	Single-stranded or double-stranded
Before interphase	46	Single stranded (46 chromatids)
After DNA is copied during interphase	46	Double-stranded (92 chromatids)
After meiosis I	23	Double-stranded (46 chromatids)
After meiosis II	23	single-stranded

Mendelian Inheritance (Gregor Mendal)

The transmission of genetic traits from parents to offspring through the passing of genes. Genes are the unit of inheritance that allows living things to pass on traits to their offspring.

Alleles

Different versions of a gene. Eg. green and yellow seeds.

Dominant Traits

Always expressed when individual has one dominant allele.

Recessive Traits

Only show if there are no dominant alleles present. If there is a dominant allele present, the trait is hidden.

Phenotype

Observable traits in an organism.

Genotype

A pair of alleles that influence the appearance of a trait. Eg. Ff

Homozygous

A genotype where the alleles are the same.

Heterozygous

A genotype where the alleles are the different.

Pedigrees

A pedigree is a “family tree” that we use to track traits throughout generations.

Pedigree Symbols

Atoms

• The smallest particle of an element.

• Composed of a nucleus with protons, neutrons and electrons, and outer shells containing electrons.

Subatomic Particles

Protons (+) Neutrons, Electrons (-) 

Subatomic Particle Masses

Electrons are 1/840 of a proton. Neutrons and protons are heavier with a similar mass.

Mass Number

The weight of neutrons and protons.

Atomic Number

Number of protons (identifies the element).

Electrons are the same number of protons (usually)

Elements

Substances made from the same kind of atom

Molecules

Substances that are chemically bonded to the same kind of atom.

Compounds

Substances that are chemically bonded to different kinds of atoms.

Groups

↓Number of valence electrons (electrons in the outermost shell).

Tells the reactivity of an element.

Periods

→Number of shells

Periodic Table Trends

Electronic Configuration

Shell	Maximum number of electrons
1st	2
2nd	8
3rd	8 (18 rarely)
4th	32

Ions

Charged particles, so they are gaining/losing electrons

Anion

If an atom gains an electron, the overall charge is negative, making it becomes an ANION

Cation

If an atom loses an electron, the overall charge is positive, making it a CATION

Isotopes

• An element that has the same atomic number but a different number of neutrons, therefore, the mass number is different. (Number of neutrons= Mass number- atomic number)

• The more neutrons, the more unstable the isotope becomes

Ionic Bonding

• Ionic bonds form between a metal and a non-metal when one or more electrons are transferred from one atom to another.

• The transfer of electrons from one atom to another results in two ions with opposite charges.

• The attraction between these opposite charges is what makes an ionic bond.

What happens to metal and non-metal atoms when bonding?

A metal loses electrons to form a cation, with a positive charge. A non-metal atom gains electrons to form an anion with a negative charge.

Chemical Bonding

Atoms are joined together by attractive forces called chemical bonds to form larger structures: molecules or lattices. Molecules are fairly small groups of bonded atoms such as oxygen (O₂) or water (H₂O). Lattices are continuous arrangements of bonded atoms in regular patterns, found in metals like iron (Fe) and crystals like quartz (SiO₂).

Why do atoms form chemical bonds?

Atoms form chemical bonds to obtain full valence shells. By bonding together in chemical reactions, atoms can reach a more stable state. 

Solute

The substance that dissolves in a solvent to produce a homogeneous mixture.

Solvent

A liquid substance capable of dissolving into one or more other substances.

Solubility

The ability of solute to dissolve in solvent and form a solution.

Law of Conservation of Mass

The law of conservation of mass states that in a chemical reaction mass is neither created nor destroyed.

Independent Variable

The factor that is changed.

Dependent Variable

The factor that is measured.

Controlled Variable

The factors that are the same.