Science Term 1 Biology

Somatic cells

Somatic cells

Body cells. These are diploid cells, meaning they have two sets of chromosomes (one from each parent).

Gametes

Sex/reproductive cells located in the gonads (testes and ovaries). Sperm in men and ova in women. These are haploid cells, meaning they have a single set of chromosomes.

Diploid number

2n. The presence of two complete sets of chromosomes (one set from each parent).

Haploid number

N. The presence of one complete set of chromosomes (only in sex cells)

Karyotype

A diagrammatic representation of the chromosomes of an organism.

Autosomes

Chromosomes that control the body characteristics of an organism. In humans, chromosomes 1-22. Autosomes control the inheritance of all an organism's characteristics except the sex-linked ones, which are controlled by the sex chromosomes.

Sex chromosomes

Chromosomes that determine the sex of an organism. In humans, women provide the X chromosome, and men can provide either another X or a Y. If it is XX, it is a female. If it is XY, it is a male.

Homologous chromosomes

A set of one maternal and one paternal chromosome that pair up with each other inside a cell. Chromosome 1 pairs with the other Chromosome 1 etc.

Sexual reproduction

When a gamete with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes. Produced by two parents.

Asexual reproduction

A mode of reproduction in which a new offspring is produced by a single parent

Prokaryotic cell

Single celled organisms that lack a nucleus and membrane-bound organelles.

Eukaryotic cell

Multi-celled organisms that contain membrane-bound organelles such as a nucleus ( and mitochondria (powerhouse of the cell)

Mitosis

A type of cell division that produces genetically identical daughter cells. Used for asexual reproduction and growth and repair in body cells. The chromosome number of the parent cell is the same as the daughter cell. PMAT x 1 (one division).

1. The DNA replicates in interphase. That means there are still the same amount of chromosomes but double the amount of chromatids.

2. In metaphase, the chromosomes align at in the middle

3. In anaphase, the sister chromatids are ‘ripped’ apart or seperated into two cells.

Meiosis

A type of cell division that occurs in gonads (testes and ovaries) that produces gametes (sex cells: sperm and egg). The four daughter cells produced contain half the chromosome number (haploid) of the original parent cell). Used for sexual reproduction and produces non-identical cells. PMAT x2 (two divisions).

1. The DNA replicates in interphase.

2. The homologous chromosomes pair up and align at the metaphase middle.

3. The DNA crosses over, causing genetic variation)

4. The homologous chromosomes separate in anaphase 1 (the sister chromatids stay together)

5. This means that it goes from 2N to N. For example, there are 4 chromosomes after DNA replication. When these homologous chromosomes split, each cell (there are 2 cells now) has 2 chromosomes.

6. Then, the sister chromatids separate into 4 cells.

Chromosome

A thread-like structure of tightly wound DNA located inside the nucleus of cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA).

DNA

Deoxyribonucleic acid (abbreviated DNA) is the molecule that carries genetic information for the development and functioning of an organism. DNA is a double-stranded molecule that forms a ‘double helix’ shape, like a twisted ladder. DNA is a polymer (poly means ‘many’) as it is made up of numerous subunits called nucleotides.

Gene

Segments of your DNA. A length of chromosome. Genes code for proteins.

Nucleotide

The basic building block of nucleic acids (RNA and DNA).

A nucleotide of DNA has three major components: • a five-carbon deoxyribose sugar • a nitrogenous base – one of adenine (A), guanine (G), cytosine (C) or thymine (T), which pair up (AT, GC) • a negatively charged phosphate group.

Chargaff’s Rule

Chargaff's rules state that in the DNA of any species and any organism, the amount of guanine should be equal to the amount of cytosine and the amount of adenine should be equal to the amount of thymine.

Alleles

Different versions of the same gene (which in itself is a segment of DNA).

Proteins

Molecules made up of amino acids. They all have different functions and play a critical part in the body

Amino acids

Molecules that combine to form proteins.

Codons

A codon is a DNA sequence of three nucleotides that codes for a particular amino acid or signalling the termination of protein synthesis (stop signals). There are 64 different codons: 61 specify amino acids and 3 are used as stop signals.

mRNA

mRNA produces proteins. It is a single-stranded RNA that codes for protein production. The coding strand, being double stranded, is unzipped. It then makes mRNA, which is single-stranded, in accordance to how it pairs up with the original DNA base. The mRNA then moves out of the nucleus and into the cytoplasm, then into the ribosomes. The ribosomes then read the mRNA and produce the proteins accordingly.

Mutation

Alterations in the DNA of an organism.

Gene mutation

A small change where the DNA base sequence in a gene is altered.

Chromosome mutation

A large change where all or part of a chromosome is deleted or duplicated. This changes the structure of the chromosome.

Spontaneous mutation

Natural, random changes in the base sequence of DNA that arise due to errors in DNA synthesis when DNA is being replicated before cell division.

Induced mutation

Accidental or deliberate exposure to environmental factors that cause a change in the DNA. Agents that induce mutations are called mutagens. These may be radiation, carcinogens, viruses etc.

Neutral (silent) mutations

Mutations that are hard to detect since they produce little or no change and little or no effect on the survival capacity of an organism.

Same sense mutation

Where a change in the 3rd base of a codon still codes for the same amino acid.

Germline mutations

A change in the DNA of a germ cell (gamete) that may be passed onto offspring.

Somatic mutations

A change in the DNA of a somatic cell that cannot be passed onto offspring

Point (gene) mutations

When there is a change in the sequence of bases of a single gene that may produce a new allele of the gene. These mutations involve 1 nucleotide. The new DNA sequence may or may not result in a new sequence of amino acids. There are 3 types: substitution of a base (results in a silent, missense or nonsense mutation), insertion of a base (results in a reading frameshift), or deletion of a base (results in a reading frameshift.

Missense substitution

When a single base is substituted for another, usually resulting in coding for a new amino acid. It may end up as a same sense mutation, meaning it still codes for the same amino acid.

Nonsense substitution

When a single base is substituted for another, but results in a triplet that does not code for an amino acids (eg. a STOP or START codon).

Reading frameshift

When a single base is inserted or deleted, meaning the triplet sequence will be shifted and therefore completely different.

Dominant

When an allele completely masks the effect of the recessive gene (there are 2 of the same gene in a pair of homologous chromosomes, one from the maternal parent and one from the fraternal parent). Represented by a capital (eg. Gg, GG). In a pair of homologous chromosomes that have one recessive and one dominant allele, the dominant allele will mask the effects of the recessive allele.

Recessive

When an allele can be masked by the dominant gene. The dominance of one allele over the other decides the characteristics of an organism

Genotype

The type of alleles an organism has for a particular trait (the person’s unique sequence of DNA).

Phenotype

The physical appearance of an organism, the observed result of its alleles for a gene.

Homozygous

2 alleles that are the same (eg. GG, or gg)

Heterozygous

2 alleles that are different (eg. Gg)

Homozygous dominant

Two dominant alleles (eg. GG).

Homozygous recessive

Two recessive alleles (eg. gg)

Heterozygous

2 different alleles (eg. Gg). In this case, the dominant masks the effects of the recessive allele.

Sex-linked inheritance

When a gene is linked to one sex chromosome only (like, if it’s linked to the X chromosome). This means that it will be more common in males because for it to be present in females they would need the gene on both chromosomes, whereas the male would only need one.

Evolution

The cumulative changes in the characteristics of living organisms from generation to generation, resulting in the development of new types of organisms over long periods of time.

Mechanisms of evolution

Sexual reproduction (crossing over), mutation, gene flow (certain alleles migrating to certain places), gene drift (a chance event completely changing the makeup of phenotypes in a place, for example a natural disaster), and natural selection (survival of the fittest: variation comes to occur due to crossing over, some of these variations mean that some are better suited to survive in that environment than others, then these organisms are more likely to reproduce, and ‘take over’).

Chiasma

The point of contact between chromosomes during meiosis where two chromatids interchange corresponding segments.

Speciation

When a group within a species separates from other members of its species and develops its own unique characteristics.

Independent assortment

The law that the allele a cell receives is completely random (it’s completely random which gene goes into which cell)

Recombination

Another word for ‘crossing over’. When the homologous chromosomes meet up and exchange genetic information

Selection pressures

Any cause that reduces or increases reproductive success in a portion of a population.

Adaptations

Heritable traits that allow an organism to survive in its habitat better.

Structural adaptations

Adaptations that affect the physicality of an organism, the body structure, anatomy (more outside than inside processes). For example, colours, size, shape, body parts (like dolphin fins)

Functional adaptations

Adaptations that affect how the organism works inside (how it operates). For example, how it digests, how it reproduces.

Behavioural adaptations

Adaptations that affect how the organism behaves. For example, nocturnal behaviour, migration in cold weather. Can be instinctive or learnt.

4 steps of natural selection

1. Genetic variation exists in the population due to sexual reproduction (crossing over and independent assortment) and mutation.

2. These genetic variations positions the organism at a high chance/ability to survive to reproduce.

3. They reproduce and pass on traits to some offspring.

4. Eventually this trait becomes more common in the population.