OCR A2 Biology Unit 2 Module 1
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edwarp Plus on July 12, 2009
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OCR Biology A2 Unit 2 Module 1
Keywords and definitions up to 2.1.10
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66 terms
Terms | Definitions |
|---|---|
 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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