WJEC Biology for AS: Student Bk

Knowledge check 1. An animal cell measured 0.035 × 10 -3 m in diameter. Convert its diameter into µm. 2. A xylem vessel measured 295000 µm in length. Convert its length to metres, expressed in standard form. 3. A bacterium measured 0.00000285 m in length. Convert its length to µm. 4. What is the diameter in nm of a DNA molecule if it is 0.002 µm across? 9 Translation is described on p100. Link The structure of nucleic acids is described on p92. Link Study point Refer to the ‘nuclear envelope’ rather than the ‘nuclear membrane’ because the nucleus is surrounded by two membranes. Units SI stands for Système Internationale, the system that defines which units are used for scientific communication. These are SI units of length: Measurement Symbol Number per metre Number of metres Object measured kilometre km 0.001 10 3 ecosystems metre m 1 1 larger organisms millimetre mm 1000 10 -3 tissues micrometre or micron µm 1 000 000 10 -6 cells and organelles nanometre nm 1 000 000 000 10 -9 molecules They are easy to remember: 1000 nm = 1 µm 1000 µm = 1 mm 1000 mm = 1m 1000 m = 1 km Nucleus This is the most prominent feature in the cell. It is usually spherical and 10–20 μm in diameter. It contains DNA which, with protein, comprises the chromosomes. The chromosomes direct protein synthesis because they are the site of transcription. The DNA also provides a template for DNA replication. Electron micrograph of the nucleus The nucleus has a number of components: ▪ ▪ It is bounded by two membranes, called the nuclear envelope , with pores which allow the passage of large molecules, such as mRNA, and ribosomes. The outer membrane is continuous with the endoplasmic reticulum. ▪ ▪ The granular material in the nucleus is nucleoplasm . It contains chromatin , which is made of coils of DNA bound to protein. During cell division, chromatin condenses into chromosomes. ▪ ▪ Within the nucleus are one or more small spherical bodies, each called a nucleolus . They are the sites of formation of rRNA, a constituent of ribosomes. WJEC AS Level Biology: Chapter 2 32