biology4alevel600

6.1 Structure and replication of  DNA 6.2 Protein synthesis  Nucleic acids have roles in the storage and retrieval of genetic information and in the use of this information to synthesise polypeptides. DNA is an extremely stable molecule that cells replicate with extreme accuracy. The genetic code is used by cells for assembling amino acids in correct sequences to  make polypeptides. In eukaryotes this involves the processes of transcription in the nucleus to produce  short-lived molecules of messenger RNA followed by translation in the cytoplasm. Learning Outcomes Candidates should  be able to: 6.1 Structure and replication of  DNA Understanding the structure  of nucleic acids allows an  understanding of their role  in the storage of genetic  information and how that  information is used in the  synthesis of proteins. a) describe the structure of nucleotides, including the  phosphorylated nucleotide ATP (structural formulae are not  required) b) describe the structure of RNA and DNA an

• Structure and replication of DNA • Role of DNA in protein synthesis Learning Outcomes Candidates should be able to: (a) describe the structure of RNA and DNA and explain the importance of base pairing and the different  hydrogen bonding between bases (includes reference to adenine and guanine as purines and to  cytosine, thymine and uracil as pyrimidines. Structural formulae for bases is not required but the recognition that purines have a double ring structure and pyrimidines have a single ring structure should  be included); (b) explain how DNA replicates semi-conservatively during interphase; (c) state that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms  part of a DNA molecue and state that a mutation is a change in the sequence that may result in an  altered polypeptide; (d) describe the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide  with reference to the nucleotide sequence for HbA (normal) a

1. Growth of a multicellular organism is a result of parent cells dividing to produce genetically identical daughter cells. 2. During cell division the nucleus divides first, followed by division of the whole cell. 3. Division of a nucleus to produce two genetically identical nuclei is achieved by the process of mitosis. 4. Mitosis is used in growth, repair, asexual reproduction and cloning of cells during an immune response. 5. Although a continuous process, mitosis can be divided for convenience into 4 phases: prophase, metaphase, anaphase and telophase. The phase between successive nuclear and cell divisions is called interphase. Replication of DNA takes place during interphase so that the new cells will each have identical DNA. 6. The period from one cell division to the next is called the cell cycle. It has four stages or phases: G1 is a growth stage, S (for synthesis) is when the DNA replicates, G2 is a second growth stage, and nuclear and cell division. G1, S and G2 are collecti

Each cell contains genes that help to control when it divides . Cells divide by mitosis only when required . When receives signals from neighbouring cells, it responds by dividing or not dividing. If this control goes wrong, cells may not divide when they should (growth does not take place, or wounds do not heal) or they may divide when they should not (a tumour may form). 1. Cancer and uncontroled cell division   In cancer: genes that control cell division mutate --> cell divide over and over again, forming an irregular mass of cells. In malignant tumour: some of cancer cells may break off and start to form new tumours elsewhere in the body. Several genes must mutate before a cell becomes cancerous. This can happen just by chance.  The risk is increased by factors that cause mutation (carcinogenic factors):          - ionising radiation (from X ray and radioactive sources emitting  α, β or γ  radiation)          -  ultraviolet radiation (in sunlight)          - chemicals

Mitosis is a nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosome. Significance of mitosis  production of geneticlly identical cells : It keeps the chromosome number constant and genetic stability in daughter cells, so the linear heredity of an organism is maintained. growth:  a single cell divides repeatedly to produce all the cells in the adult organism repair of tissue and  cell replacement:  produce new cells to replace ones that have been damaged ( repair and generation of lost parts)  or worn out ( healing of wouds). asexual reproduction:  a single parent gives rise to genetically identical offspring Strictly speaking, mitosis is division of the nucleus of the cell. After this, the cell itself usually divides as well ( cytokinesis ). The cell cycle The cell cycle is the continuous cycle of  growth  and  mitotic division . It has 2 major phases:  Interfase and Miotic phase. 1. Interphase

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each  chromosome  is made up of  DNA  tightly coiled many times around proteins called  histones  that support its structure. Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during  cell division  and is then visible under a microscope.  Each chromosome has a constriction point called the  centromere,  which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes. A  chromatid  is 1 of the 2 identical strands of DNA that make uo a chromosome. 2 chromatids are joined by a centromere. Befor

5.1 Replication and division of nuclei and cells      5.2 Chromosome behaviour in mitosis When body cells reach a certain size they divide into two. Nuclear division occurs first, followed by division of the cytoplasm. The mitotic cell cycle of eukaryotes involves DNA replication followed by nuclear division. This ensures the genetic uniformity of all daughter cells. Learning Outcomes Candidates should  be able to: 5.1 Replication and division of nuclei and cells  During the mitotic cell cycle, DNA is replicated and passed to daughter cells. Stem cells in bone marrow and the skin continually divide by mitosis to provide a continuous supply of cells that differentiate into blood and skin cells. a) describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres b) explain the importance of mitosis in the production of genetically identical cells, growth, cell replacement, repair of tissues and asexual reproduction c) outline the cell cycle,

• Replication and division of nuclei and cells • Understanding of chromosome behaviour in mitosis Learning Outcomes Candidates should be able to: (a) explain the importance of mitosis in the production of genetically identical cells, growth, repair and  asexual reproduction; (b) outline the cell cycle, including growth, DNA replication, mitosis and cytokinesis; (c) [PA] describe, with the aid of diagrams, the behaviour of chromosomes during the mitotic cell cycle and  the associated behaviour of the nuclear envelope, cell membrane, centrioles and spindle (names of the  main stages are expected); (d) explain how uncontrolled cell division can result in the formation of a tumour and identify factors that can  increase the chances of cancerous growth; (e) explain the meanings of the terms haploid and diploid (see section 5) and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction (note: descriptions of homologous chromosomes are not required for AS Lev

1 The basic structure of a membrane is a 7 nm thick phospholipid bilayer with protein molecules spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the layers. Hence the structure is described as a fluid mosaic – the scattered protein molecules resemble pieces of a mosaic. 2 Phospholipid bilayers are a barrier to most watersoluble substances because the interior of the membrane is hydrophobic. 3 Cholesterol is needed for membrane fluidity and stability. 4 Some proteins are transport proteins, transporting molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape. 5 Some proteins act as enzymes – for example, in the cell surface membranes of microvilli in the gut. 6 Glycolipids and glycoproteins form receptors – for example, for hormones or neurotransmitters. They also form antigens, which are cell recognition markers. 7 The cell surface mem