Genetic Test - Revision

Year 10 Science - Genetics Study Notes

Overview of Topics

• In this topic you will learn about:

  • DNA the molecule: DNA and chromosomes

  • Reading the genetic code: Transcription and translation

  • Cell division: Mitosis and meiosis

  • Characteristics and inheritance: alleles, pedigrees, mutations

  • Using genetics: genetic testing, genetic modifications

DNA the Molecule

Definition of DNA

• DNA = Deoxyribonucleic Acid

• It is a molecule that contains all the instructions for every job performed by the cell. This information can be passed from one generation to the next.

• It contains a code unique to all individuals.

• Every cell in your body (except red blood cells) contains the same DNA.

Composition of DNA: Nucleotides

• DNA is made up of individual parts called nucleotides which are linked together.

• A nucleotide is made of:

  • A nitrogenous base (often just called a base)

  • A sugar molecule (deoxyribose)

  • A phosphate group

DNA Structure

• In DNA, there are four bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).

• The structure of DNA is a long chain of nucleotides forming a sugar-phosphate backbone. The nucleotides are attached to each other, with the phosphate of one nucleotide attached to the sugar of the next nucleotide.

• The bases are held together by weak hydrogen bonds and link in specific ways (called complementary base pairing).

  • A — T and T — A

  • C — G and G — C

Properties of DNA

• If the two strands of DNA unwind, each strand can be used to make a new DNA molecule.

• The order of the bases is a code for making proteins, which are important molecules involved in the structure, function, and regulation of the body’s tissues and organs.

The Double Helix Structure

• Complementary base pairing results in two strands winding into a double helix shape (like a twisted ladder).

Example Question

• A DNA strand has the following bases: GCTTAC. What are the bases on its complementary strand?

  • Possible answers:

  • a) GCTTA C

  • b) CGAATG

  • c) GAATAG

  • d) ATCCGT

Chromosomes

Definition and Structure

• Chromosomes are made up of DNA tightly wrapped around proteins.

• They are only visible when cells are dividing.

• A single chromosome equals a molecule of DNA (a DNA double helix).

• In a human cell nucleus, there are 46 chromosomes: 23 from the mother and 23 from the father.

• Gametes (sex cells) contain half the number of chromosomes as body cells (23 chromosomes, haploid).

Understanding Chromosomes

• Chromosomes can be organized into a karyotype, which is a picture of all homologous (matching pairs of chromosomes) arranged from largest to smallest.

• The sex of an individual is determined by the 23rd chromosome pair: XX for female and XY for male.

• A duplicated chromosome consists of two joined strands, called sister chromatids, connected at the centromere.

Cell Division

Mitosis

• Mitosis is a process of cell division that results in two genetically identical daughter cells. It occurs in somatic cells (body cells) and allows for growth and repair.

• Somatic cells only start mitosis when new cells are needed.

The Cell Cycle

• Cells spend most of their time in interphase, during which normal functioning occurs (growth, cell processes).

• Mitosis does not change the number of chromosomes (humans maintain 46 chromosomes).

• Before mitosis, a cell duplicates its DNA to produce duplicated chromosomes.

Stages of Mitosis

1. Prophase: Chromosomes appear, nuclear membrane disappears, spindle forms.

2. Metaphase: Chromosomes line up in the center of the cell.

3. Anaphase: Chromatids separate at the centromere and move to opposite poles.

4. Telophase: Nuclear membranes reform, followed by cytokinesis where the cytoplasm divides producing two daughter cells.

Cancer and Mitosis

• Mitosis is carefully regulated; errors can lead to apoptosis (programmed cell death).

• Damage to DNA from radiation, viruses, or chemicals (mutagens) can lead to uncontrolled growth, potentially resulting in cancer.

Meiosis

• Meiosis is a process of cell division resulting in the formation of gametes with half the genetic material of the parent. Gametes are haploid (n).

• Most somatic cells in a human are diploid (2n, 46 chromosomes). When haploid sperm and egg unite at fertilization, a diploid cell is produced.

Stages of Meiosis

• Meiosis I: Chromosomes are duplicated, then homologous chromosomes separate.

• Meiosis II: Similar to mitosis, but results in four haploid daughter cells.

Comparison of Mitosis and Meiosis

Characteristics and Inheritance

Alleles

• An allele is a version of a gene at the same position (or loci) of a chromosome. A person inherits two alleles for each gene, one from each parent.

• Homozygous: two identical alleles for a trait.

• Heterozygous: two different alleles for a trait.

Trait Types

• Dominant Trait: needs only one copy of an allele to be expressed; represented by a capital letter.

• Recessive Trait: only expressed when two copies are present; represented by a lowercase letter.

• Carrier: has the allele for a recessive trait but does not express it.

• Phenotype: physical expression of a trait resulting from interaction between genes and the environment.

• Genotype: combination of alleles for a particular trait.

Punnett Squares

• A Punnett square is a graphical way to show how single traits are passed on.

• Example: Tongue rolling trait, where:

  • Dominant (tongue roller): R

  • Recessive (non-tongue roller): r

• If a homozygous non-tongue roller (rr) has children with a heterozygous tongue roller (Rr), the offspring probabilities are:

  • 50% tongue rollers (Rr)

  • 50% non-tongue rollers (rr)

Co-Dominance

• Co-dominance occurs when two different alleles can both appear in the phenotype, such as human blood type AB where both A and B molecules are present.

Sex-Linked Traits

• Sex chromosomes determine the sex of an organism: Females have XX and Males have XY.

• Traits linked to sex chromosomes (e.g., color blindness) are more commonly expressed in males due to having only one X chromosome.

Pedigrees

• Pedigrees are graphical representations showing phenotypes of individuals and their ancestors over generations (family tree). Males are squares, females are circles; shaded symbols denote phenotypic expression.

Mutations

• A mutation is a permanent change in the DNA sequence that can be passed to daughter cells.

  • Point Mutations: involve a change in a single base (substitution, insertion, deletion).

  • Chromosomal Mutations: involve changes to the number or structure of chromosomes, often due to non-disjunction.

Using Genetics

Genetic Testing and Screening

• Genetic testing identifies mutations in genes using probes, which are short DNA sequences that bind to the mutated gene.

• Common tests include maternal serum screening for Down syndrome, and newborn screening for diseases like cystic fibrosis.

Genetic Modification

• Genetic modification involves changing an organism's DNA. This includes inserting genes from other organisms to produce desired traits (e.g., drought-resistant plants, Bt cotton).

• Transgenic Organisms: are those that have had genes from another organism inserted into their chromosomes.

Gene Therapy

• Gene therapy is the insertion of a healthy allele into an organism to treat diseases. Therapeutic gene therapies target somatic cells, while germ-line gene therapies affect gametes and are currently illegal in Australia.

Stem Cells

• Stem cells are undifferentiated cells that can mature into various specialized cells. Types include:

  • Pluripotent: embryonic stem cells that can differentiate into most body cell types.

  • Multipotent: adult stem cells that differentiate into specific cell types.

Ethical Considerations in Stem Cell Research

• The creation of embryos for stem cell collection raises ethical issues, particularly concerning life potential. Only 'excess embryos' from IVF are typically used for research.

Cell Division

Mitosis

• Mitosis is a process of cell division that results in two genetically identical daughter cells. It occurs in somatic cells (body cells) and allows for growth and repair.

• Somatic cells only start mitosis when new cells are needed.

The Cell Cycle

• Cells spend most of their time in interphase, during which normal functioning occurs (growth, cell processes).

• Mitosis does not change the number of chromosomes (humans maintain 46 chromosomes).

• Before mitosis, a cell duplicates its DNA to produce duplicated chromosomes.

Stages of Mitosis

1. Prophase: Chromosomes appear, nuclear membrane disappears, spindle forms.

2. Metaphase: Chromosomes line up in the center of the cell.

3. Anaphase: Chromatids separate at the centromere and move to opposite poles.

4. Telophase: Nuclear membranes reform, followed by cytokinesis where the cytoplasm divides producing two daughter cells.

Cancer and Mitosis

• Mitosis is carefully regulated; errors can lead to apoptosis (programmed cell death).

• Damage to DNA from radiation, viruses, or chemicals (mutagens) can lead to uncontrolled growth, potentially resulting in cancer.

Meiosis

• Meiosis is a process of cell division resulting in the formation of gametes with half the genetic material of the parent. Gametes are haploid (n).

• Most somatic cells in a human are diploid (2n, 46 chromosomes). When haploid sperm and egg unite at fertilization, a diploid cell is produced.

Stages of Meiosis

• Meiosis I: Chromosomes are duplicated, then homologous chromosomes separate.

• Meiosis II: Similar to mitosis, but results in four haploid daughter cells.

Comparison of Mitosis and Meiosis