Biology Semester one Exam

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DNA: Information molecule that is the universal basis of an organism’s genetic material

• It contains instructions, written in a chemical code, for the production of proteins for the cell.
• The two strands of a DNA double helix link by hydrogen bonds between complementary bases.
• A and T link with two hydrogen bonds, G and C link with three hydrogen bonds.

Nitrogenous Bases: 

• Adenine (A)
• Thymine (T)
• Guanine (G) d
• Cytosine (C)

Difference between DNA and RNA:

DNA:

• Contains the sugar deoxyribose
• Double stranded
• Stores and transfers genetic information
• Adenine, guanine, cytosine and thymine

RNA:

• Contains the sugar ribose
• Single stranded
• Messenger between the DNA and ribosomes to make proteins
• Adenine, guanine, cytosine and URACIL

DNA Replication: DNA replicates by a semi-conservative mechanism where one of the strands in the newly formed molecule is new and the other is the original strand.

DNA Helicase: An enzyme that helps the two strands of the DNA double helix unwind and separate.

Replication Fork: The junction between the unwound single strands of DNA and the intact double helix.

DNA Polymerase: An enzyme capable of making exact copies of fragments of DNA.

DNA Ligase: An enzyme used to catalyse the formation of a bond between two pieces of DNA.

1. Helicase enzyme easily breaks the weak hydrogen bonds between the nucleotides to expose them, which unzips the double helix.
2. Replication fork moves along double helix continually unzipping it.
3. Within the nucleus, thousands of free nucleotides attach to the exposed nucleotides, because of the polymerase enzyme.
4. Ligase enzyme seals the new short stretches of nucleotides into a new strand.
5. Outcome= 2 double stranded helix coming from 1 parent strand = semi conservative replication.

Genomics: Study of genomes.

Genomes: Sum of the entire DNA in the cell of an organism.

Reproduction: Occurs in the gametes (reproductive cells).

• Occurs through mitosis, meiosis and binary fission.

Asexual: Offspring are produced from a single parent.

Sexual: Offspring are produced from two parents.

Chromosomes of Eukaryotes: DNA and associated proteins make chromosomes.

• Somatic or body cells are diploid, containing 23 pairs of chromosomes; one chromosome of a pair comes from the male parent and the other from the female parent.

Organelles: Specialised part of a cell, with its own specific function.

Chromatin: A complex of proteins and DNA in eukaryotic chromosomes.

Chromosome: Structure composed of DNA and proteins.

• Along its length, it contains linear arrays of genes carrying genetic info.
• Prokaryotes have one circular chromosome.
• Eukaryotes have multiple linear chromosomes.

Histone: A protein around which DNA winds in eukaryotic cells.

Centromere: The waist-like constriction in a chromosome required for the movement of chromosomes during cell division.

Homologous: Pair of chromosomes that have the same size, shape and genes at the same locations.

Autosomes: Chromosomes that are the same in both males and females of a species; they do not include sex chromosomes.

Heterosomes: Non-identical chromosomes pairing up at meiosis.

(Sex Chromosome): Affects sexual traits.

• One sex has homologous sex chromosomes
• The other sex has a dissimilar set of chromosomes.

Locus: Position a gene occupies in a chromosome.

Allele: One of different genes, at the same locus, determined by small differences in the DNA sequence of the gene.

Gene: Unit of hereditary that transmits info from one generation to the next; a segment of DNA that codes for polypeptide.

Chromosomes of Prokaryotes: Chromosomes in prokaryotic cells are generally circular and are similar to chromosomes found in the mitochondria and chloroplasts.

Cell Division: Eukaryotic cell division involves a number of phases resulting in nuclear division (mitosis and meiosis) and cytoplasmic division (cytokinesis).

Cell Cycle: 

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Asexual Reproduction: One part gives rise to new individual from its body cells.

• Offspring are genetically identical.

Sexual Reproduction: Meiosis produces varied haploid cells, fertilization restores diploid number.

• Each gamete has its own unique combination of genes.

Variation:

• Independent assortment: Chromosomes randomly separated to opposite ends of cell during meiosis.

• Chance event leading to different gametes having different chromosomal combinations.

• Crossing over during meiosis: Two homologous chromosomes exchange segments, then separate and each have new combination of alleles.

• Resulting nuclei will have chromosomes not identical to the original nucleus.

• Incomplete chromosome separation
• Random mating: Leads to variation in offspring due to random fusion of gametes.

Mitosis: Sequence of steps to produce two diploid daughter cells from a diploid parent cell.

Interphase: 

• Chromosomes not visible, can’t be distinguished under light microscopes.
• Just before interphase ends, centrioles are visible and chromatin threads become visible under light microscope.

Prophase: 

• Chromatin threads condense and become visible as double strands consisting of two chromatids, held together by a centromere.
• Spindle forms made from microtubules, originating from centrioles.
• Nucleolus disappears.
• Nuclear membrane breaks down.

Metaphase:

• Chromosomes from out of centre of cell and line up along the equator.

Anaphase: