Biology shorts 3.1 (2).docx

3.1 === **Cell differentiation** -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------...

3.1 === **Cell differentiation** -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **With the exception of sperm and ova, the cells of a multicellular organism are genetically identical. And yet the structure of them varies considerably. How is it possible for cells with the same genetic information to be so different in structure, and why is there so much variation in the structure of cells in a multicellular organism?** -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Unicellular and multicellular** All living organisms consist of at least one cell. One-celled ones are called **unicellular** **organisms** e.g. bacteria, unicellular fungi like yeasts and single-celled aquatic organisms like phytoplankton. Organisms made up of many cells are called **multicellular organisms** e.g. animals, plants and fungi like mushrooms. **Chromosomes, DNA and genes** **DNA** is a molecule that carries genetic information. Different sections or lengths of DNA are called **genes**. Different genes carry different genetic information e.g. the instructions to make different **protein** molecules. For most of the life of a cell from a multicellular organism, its DNA consists of long, loose strands located in the nucleus called **chromatin**. Just before these cells divide, the chromatin coils into nucleosomes that in turn form into short, tightly packed pairs called **chromatids**. Each of these is a **chromosome** with around several hundred different genes. Figure 3.1 shows a chromosome is two chromatids, each one a package of tightly coiled DNA. Figure 3.1: Chromosome structure in a cell from a multicellular organism *(Source: University of Washington).* Chromosomes **Genetically identical cells** Cells in a multicellular organism carry the same number and type of chromosomes. This is true regardless of the type of cell e.g. human muscle, bone and nerve cells all carry 46 chromosomes, 23 types each one in a pair. Each type of chromosome consists of the same genes in the same sequence. For these reasons, the cells in a multicellular organism are said to be **genetically identical**. +-----------------------------------------------------------------------+ | **Learning requirement 1:** | | | | - Recognise that cells in a multicellular organism are genetically | | identical. | +-----------------------------------------------------------------------+ **Gene expression** If the instructions carried by one gene are used to make one specific protein, that gene is said to be **expressed**, or 'switched on'. If a gene is 'switched off', its instructions for protein-making are not used, meaning that specific protein is not made by the cell. The gene is not expressed. How many genes are expressed in any given cell varies from cell type to cell type, time of day (e.g. after eating) and the stage of development of the organism (e.g. during puberty). As a general rule, a typical human cell might express 20% of its genes at any given time. Gene expression is tightly controlled. One method involves regulatory protein molecules that bind to the DNA near a gene called **activators** and **repressors**. The binding of an activator near a gene switches it on, so the gene is expressed (its instructions are used by the cell to make one protein). When a repressor binds near a gene, however, it switches the gene off, so the gene is not expressed. **Cell specialisation** The cells in a multicellular organism are **specialised**. This means multicellular organisms consist of different cell types whose unique structure permits them to perform a specific function. Figure 3.2 shows specialised cells have a unique structure to function differently. This is not because they carry different genes but because they are the product of gene expression occurring differently. In other words, during development, cells have certain genes switched on whilst others are switched off. This enables them to make a specific group of proteins that allow them to grow in a specific way, leading to a unique structure. This process by which genetically identical cells form into cells with a specific structure and function is called **cell differentiation**. Figure 3.2: Some specialised animal cells (left) and (right) some specialised plant cells *(Source: SlidePlayer).*  To learn about cell specialisation and cell differentiation view the clip below: +-----------------------------------------------------------------------+ | **Learning requirement 2:** | | | | - Recognise that gene expression is responsible for cell | | specialisation. | +-----------------------------------------------------------------------+ **SCIENCE UNDERSTANDING CHECK** 1\. Specific cell structure and functions develop through cell differentiation. **TEST YOUR LEARNING *For selected answers see Topic 3 Assessment for learning on immi*** 1\. Which one of the following statements is ***not*** correct? J. Chromosomes have DNA. K. Chromosomes carry genes. L. Chromatin is tightly coiled DNA. M. Genes carry information cells can use to make proteins. \[1 mark\] 2\. Which one of the following statements is correct? J. Different types of human cell carry different numbers of chromosomes. K. All specialised human cells have the same genes. L. Genes may be switched on if a repressor binds to DNA near them. M. Regulatory protein molecules can only switch genes on. \[1 mark\] 3\. Genes carry the instructions cells need to make a type of molecule. State the name of this molecule. Protein \[1 mark\] 4\. Describe what it means to say that a gene is 'switched on'. A gene is switched on when the instructions carried are used to make a specific protein \[2 marks\] 5\. The cells of multicellular organisms are specialised. State what this means. Multicellular organisms consist of different cell types which their structure permits them to a specific function. \[1 mark\] 6\. Despite being genetically identical, the cells of a multicellular organism are different in structure. Explain how this is possible. This is possible because genes are expressed differently in different cell types. The activators and repressors which are binded to the DNA turn specific genes on or off based on factors such as time of day, age, environment, etc. \[3 marks\] 7\. Explain how the structure of a red blood cell, and a leaf cell, is related to its function. The red blood cell has a biconcave which increases the surface area for the absorption of oxygen. Also the lack of a nucleus allows more space for haemoglobin, enabling more efficient oxygen transport. A leaf cell has a large surface area which maximises the absorption of light, they also contain chloroplasts which capture sunlight for photosynthesis. It's thin walls also Facilitate gas exchange. \[6 marks\] 8\. Compare the action of activators and repressors in the control of gene expression. Activators and repressors are regulatory protein molecules which bind to the DNA. The binding of an activator near a gene switches it on (expressed) whereas the binding of a repressor switches it off (not expressed). \[2 marks\] --------------------------------------------------------------------------------------------- **Additional questions: SACE1 Biology Workbook Chapter 3.1 Questions 1, 6-8 page 215, 216** ---------------------------------------------------------------------------------------------