OCR A2 Biology Unit 2 Module 1

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OCR Biology A2 Unit 2 Module 1 Keywords and definitions up to 2.1.10

Allele

An alternative version of a gene carrying sequence differences.

Apoptosis

Programmed cell death. An orderly process by which cells self-destruct in an orderly fashion after a certain number of cell divisions, or if they cannot repair DNA damage.

ATP

Molecule (nucleotide derivative) found in all living cells and involved in energy transfer. When it is hydrolysed energy is released.

Bivalent

Pair of synapsed (joined) homologous chromosomes during prophase and metaphase of meiosis I.

Central dogma

The fundamental relationship between DNA, RNA and protein first outlined by Francis Crick in 1958.

Chiasmata

The points where non-sister chromatids within a bivalent join, where they cross over.

Coding strand

The DNA strand encompassing a gene that carries the sequence identical to the message sequence.

Codominant

A characteristic where both alleles contribute to the phenotype.

Codon

A sequence of three bases that codes for a single amino acid according to the genetic code.

Crossing over

Where non-sister chromatids exchange alleles during prophase I of meiosis.

Degenerate

A term applied to the genetic code that describes the fact that more than one triplet code is used to code for all amino acids other than tryptophan and methionine.

Diploid

Having two sets of chromosomes (eukaryotic cell or organism). Denoted by 2n.

Discontinuous variation

Genetic variation where there are distinct phenotypic categories. Usually controlled by one gene to a few genes. Examples include cystic fibrosis, shape of earlobes in humans and height in pea plants.

DNA mutation

A change to the DNA structure altering the sequence of bases.

Dominant

Characteristic in which the allele responsible is expressed in the phenotype even in those with heterozygous genotypes.

Frameshift mutation

A change in the DNA within a gene involving the insertion or deletion of a number of bases that is not a multiple of three. This causes the downstream gene sequence to be scrambled because the correct reading frame is lost.

Gametes

Specialised sex cells. In many organisms the gametes are haploid and are produced by meiosis.

Gene

A length of DNA that codes for one (or more) polypeptides/proteins. Some may code for RNA or regulate other genes.

Genetic code

The way in which the 64 possible base triplets map onto the 20 amino acids that are used in polypeptide chains.

Genetic reassortment

Variation introduced due to the random orientation and subsequent segregation of maternal and paternal chromosomes during meiosis 1, or sister chromatids at meiosis 2.

Genetic variation

Variation of genetic information in a gene pool.

Genome

All the genetic information within an organism/cell.

Genotype

Alleles present within cells of an individual, for a particular trait/characteristic.

Haploid

Eukaryotic cell or organism having only one set of chromosomes. Denoted by n.

Hayflick limit

A reference to the fact that normal body cells can only divide a finite number of times, normally about 50 mitotic divisions. In contrast, tumour cells are essentially immortal.

Heterozygous

Eukaryotic cell or organism that has two different alleles for a specific gene.

Homeobox genes

Genes that control the development of the body plan of an organism.

Homeotic selector genes

These direct the development of individual body segments. They are master genes that control other regulatory genes.

Homozygous

Eukaryotic cell or organism that has two identical alleles for a specific gene.

Hox clusters

Groups of homeobox genes. More complex organisms have more such groups, probably due to a mutation that duplicated them.

Initiator codon

The triplet code recognised by the ribosome that signals the start point for translation. Almost invariably a methionine codon (ATG).

Insertion/deletion mutation

A change in DNA sequence involving the introduction or loss of 1- 1000s of bases.

Kinetochore

Structure formed on the centromere that attaches the chromosomes to the spindle fibres during mitosis and meiosis.

Locus

Specific position on a chromosome, occupied by a specific gene.

Meiosis

Type of nuclear division. A reduction division. The chromosome number is halved. It involves two divisions. It produces cells that are genetically different from each other and from the parent cell.

Missense mutation

A change in DNA sequence that leads to a change in the amino acid sequence coded for by a gene.

Monogenic

Characteristic coded for by one gene.

Morphogen

A substance that controls the pattern of tissue development. It is produced in a particular region of a developing organism. It diffuses to other cells, which then enter a specific developmental pathway.

Mutation

Structural change to genetic material - either to a gene or to a chromosome.

Neutral mutation

A mutation that even though it may change a coding sequence exerts no harmful or beneficial effects.

Non-disjunction

Failure of members of a homologous pair of chromosomes, or of a pair of chromatids, to separate during nuclear division.

Nonsense mutation

A change in DNA sequence that introduces a premature stop codon within a DNA coding sequence.

Operon

A stretch of DNA consisting of two or more genes that are transcribed together and coregulated.

Paternal chromosome

Member of a pair of homologous chromosomes that originally came from the male gamete.

Phagocytosis

Term to describe the engulfing by macrophages of vesicles from cells that have undergone apoptosis.

Phenotype

Observable characteristics of an organism.

Polarity

Refers to the location of cells with respect to the head end (anterior) or tail end (posterior) of the body.

Polypeptide

Large polymer molecule made of many amino acids joined by peptide bonds.

Polyploid

Eukaryotic organisms or cell with more than two sets of chromosomes.

Polysome

A length of messenger RNA carrying multiple ribosomes at different stages of protein synthesis.

Programmed cell death

Alternative term for apoptosis.

Protein

Macromolecule that is a polymer of many amino acids joined by peptide bonds. May comprise more than one polypeptide chains.

Proto-oncogene

Gene that can undergo mutations to become an oncogene, which induces tumour formation (cancer).

Recessive

Characteristic in which the allele responsible is only expressed in the phenotype if there is no dominant allele present.

Ribosomal RNA

Three types of RNA that form an important structural and functional role within the ribosome.

Segmentation genes

Genes that control the development of polarity within a body segment during development.

Sexual reproduction

Production of new organisms involving fusion of nuclei from male and female gametes, usually from unrelated individuals. Increases genetic variation in the population.

Silent mutation

A change in DNA sequence within a gene that does not lead to a change in the amino acid sequence due to the degeneracy of the genetic code.

Spindle fibres

Made of microtubules, these structures are responsible for providing the framework for segregation of the chromosomes during mitosis and meiosis.

Stop codon

One of three triplet codes (UGA, UAG or UAA) that causes the termination of translation by the ribosome.

Stutter mutation

Causes an expansion or contraction of a run of identical codons within a gene affecting the length of a run of identical amino acids in the polypeptide.

Template strand

The DNA strand that is used for assembly of mRNA by RNA polymerase using base-pairing rules. Complementary to the message sequence.

Transcription

The formation of an RNA molecule, using a length of DNA as a template and catalysed by RNA polymerase.

transfer RNA

A short length of about 100 RNA nucleotides that adopts a clover leaf structure and carries an amino acid to the ribosome to be incorporated into a growing polypeptide chain.

Translation

Stage of protein/polypeptide synthesis in which the amino acids are assembled at ribosomes according to the genetic code.

Bleb

Small vesicle formed that breaks away from the cell membrane during apoptosis

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