Science: Genes and DNA

adenine (A): nitrogen-rich base; forms pair with thymine (T)

alleles: different forms of the same gene located at the same point of homologous chromosomes

autosomes: all the chromosomes in a cell other than the sex chromosomes

carcinogen: a mutagen that can lead to cancer

centromere: the point on a chromosome where the two chromatids are joined together

chromatid: one of the strands of a chromosome following replication

chromosomes: thread-like structures in the nucleus; composed of DNA and proteins; contains the genetic information in the form of genes chromatids in a replicated chromosome

complementary base pairs: a pair of bases that can join to make the rungs of the DNA ladder—adenine and thymine, guanine and cytosine

cytosine (C): nitrogen-rich base; forms pair with guanine (G)

deoxyribonucleic acid (DNA): a double helix made of nucleotides; the molecule that determines the genetic characteristics of most living things

deoxyribose sugar: one of the parts that make up a nucleotide

diploid number: the number of chromosomes in body cells; two sets or 2n

DNA profiling: a pattern of DNA fragments unique to each individual

dominant trait: the trait that is observed in the outward appearance of a heterozygous individual

double helix: shape like that of a twisted rope ladder

gametes: sperm and egg cells

gene: a section of DNA that carries the genetic code for a particular characteristic

gene splicing: the process used to add a gene into or remove genes from DNA

gene therapy: the process of replacing a defective gene with a normal gene

genetically modified: having the genes changed

genetics: the study of inherited characteristics called traits

genome: the complete set of DNA including all the genes of a cell, individual or species

genotype: genetic information carried by an individual

guanine (G): nitrogen-rich base; forms pair with cytosine (C)

haploid number: the number of chromosomes in gametes; one set or n

heterozygous: having two different alleles on homologous chromosomes

homologous chromosomes: chromosomes with genes for particular characteristics at the same location

homozygous: having two identical alleles on homologous chromosomes

diploid number: the total number of chromosomes in a somatic cell; two sets or 2n

induced pluripotent stem cells (iPSCs): cells that have been made to revert to pluripotent state by the introduction of certain genes

meiosis: the type of cell division that produces gametes with half the number of chromosomes of the parent cell

mitosis: the type of cell division that produces two daughter cells identical to the parent cell

mutagen: an agent that causes a mutation

mutation: a mistake that happens as DNA is copied, causing a change to the base sequence

nitrogen-rich base: part of a nucleotide; the four types are adenine (A), guanine (G), cytosine (C) and thymine (T)

nucleotides: the building blocks of DNA; comprised of deoxyribose sugar, a

phosphate group and a nitrogen-rich base

pathogen: a disease-causing organism

pedigree: diagram used to show patterns of inheritance over generations

phenotype: observable characteristics of the individual; the way the

genotype is expressed

phosphate group: one of the parts that make up a

nucleotide

plasmid: ring of DNA found in bacteria

pluripotent embryonic stem cells: stem cells that are capable of becoming any one of the 200 or so different cell types found in the human body

pure breeding: where all individuals have the same genetic information for a characteristic generation after generation

recessive trait: the trait that remains hidden in the heterozygous condition and seen only in the homozygous condition

recombinant DNA technology: technology that allows DNA to be recombined with other genes

replication: the process of making copies of DNA

sex chromosomes: the chromosomes that determine the sex of an individual; in humans they are the X and Y chromosomes

sex-linked genes: genes present on the sex chromosomes

single nucleotide polymorphism (SNP): locations on chromosomes that differ by only one base from one human to another

thymine (T): nitrogen-rich base; forms pair with adenine (A)

traits: inherited characteristics

Watson–Crick model: the double helix structure of the DNA molecule

zygote: the cell formed by fusion of two gametes, a sperm and egg

The difference between one gene and the next is the:

• order of bases along the DNA strand

• number of bases in that section of DNA

Replication: In the first step of DNA replication, the strands of the double helix separate from each other in much the same way as a zip opens. The bases are then exposed. Within the nucleus there are individual nucleotides that are not yet part of a DNA chain. In step 2, these nucleotides pair up with the exposed bases following the rules of complementary base pairing. The sugar and phosphate molecules bond with

neighbouring nucleotides and new strands of DNA are formed. The result in step 3 is two identical DNA molecules, each one a double helix. They are also identical to the original parent DNA. Replication occurs on both of the exposed strands of DNA, and the result is two identical molecules of DNA. Each replicated chromosome has the two identical DNA molecules which are now called chromatids.

Process of Mitosis: When the cell begins to divide, the DNA coils up

and separate chromosomes become visible. Each chromosome has two chromatids. The membrane surrounding the nucleus breaks down. Chromosomes line up across

the equator (middle) of the cell and a network of fibres appears, extending from the poles (ends) of the cell to each chromosome. The chromatids separate to become two independent chromosomes. The network of fibres contracts, pulling the chromosomes to opposite poles (ends) of the cell. A new nuclear membrane encloses the chromosomes at each pole. The chromosomes uncoil and are no longer visible as individual strands. Division of the nucleus is complete. The cytoplasm then divides and the result is two identical daughter cells. The daughter cells grow in size in preparation for the next round of cell division or they

become specialised cells, such as neurons or muscle cells, that can no longer divide.

Process of Meiosis: The nuclear membrane breaks down and then, in preparation for the first part of meiosis, the homologous pairs of chromosomes line up on the equator of the cell. A network of fibres (called spindle fibres) extends from the poles of the cell to each chromosome pair (Metaphase I ). The fibres contract, drawing one chromosome from each pair to opposite poles of the cell (Anaphase I ). At this stage, each chromosome is still two chromatids (Telophase I ). A new network of fibres forms at right angles to the first. The fibres attach to the chromosomes that have lined up on the equator of the cell (Metaphase II ). This time when the fibres contract, the chromatids are pulled apart towards the poles of the cells (Anaphase II ). New nuclear membranes form and the cytoplasm divides to produce four new cells, each containing the haploid number of chromosomes. These cells are the gametes or sex cells (Telophase II ). In humans, the gametes have 23 chromosomes.