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Pages 205-207 Specification area 3.4.1 DNA and Chromosomes Pages 205-207 Specification area 3.4.1

Specification

Objectives Distinguish between the DNA in prokaryotic cells and the DNA in eukaryotic organisms Describe the structure of a chromosome Describe the nature of homologous chromosomes Explain how genes are arranged on a DNA molecule Explain what is meant by an allele

Possible activities Distinguish between the DNA in prokaryotic cells and the DNA in eukaryotic organisms Use animation to show scale of chromosomes in eukaryotic cells, and how chromosomes are made of DNA and histones. Introduce the concept of a gene. - Teacher explanation about the difference between the arrangement of DNA in prokaryotic cells and eukaryotic cells, text top p205 - Students generate a summary table comparing and contrasting prokaryotic and eukaryotic DNA http://www.yourgenome.org/video/zoom-in-on-your-genome

Prokaryote and Eukaryote DNA molecules Prokaryotes e.g. bacteria DNA is short DNA is circular DNA is not wound around proteins Plasmid DNA maybe present Eukaryotes e.g. plants, animals DNA is long DNA is linear DNA is wound around proteins called histones and then coiled tightly to make a chromosome

Objective 2 Describe the structure of a chromosome When are chromosomes visible? Once DNA has replicated how many chromatids make up a chromosome? Where are chromatids joined together? What are histones? Why is DNA packed onto histones? What can you say about the number of chromosomes in different organisms?

Objective 2 Describe the structure of a chromosome When are chromosomes visible? Once DNA has replicated how many chromatids make up a chromosome? Where are chromatids joined together? What are histones? Why is DNA packed onto histones? What can you say about the number of chromosomes in different organisms? During cell division Two Centromere Proteins DNA wrapped around in eukaryotic cells Because it is very long so needs to be condensed Different organisms have different numbers of chromosomes

Genes in Eukaryote Cells Eukaryotes have genetic information stored in chromosomes in the nucleus of each cell: Nucleus contains inherited information: The total collection of genes located on chromosomes in the nucleus has the complete instructions for constructing a total organism. Cytoplasm: The nucleus controls cell metabolism; the many chemical reactions that keep the cell alive and performing its designated role. Nucleus Structure of the nucleus Nuclear pores are involved in the active transport of substances into and out of the nucleus Nucleolus is involved in the construction of ribosomes Nuclear membrane encloses the nucleus in eukaryotic cells Chromosomes are made up of DNA and protein and store the information for controlling the cell

Genes Outside the Nucleus in Eukaryote Cells Ribosome Mitochondrial DNA Eukaryotes have two types of organelles with their own DNA: mitochondria chloroplasts The DNA of these organelles is replicated when the organelles are reproduced (independently of the DNA in the nucleus). Mitochondrion Chloroplast Chloroplast DNA

Mitochondria and chloroplasts Have their own DNA Short Circular No histones Just like prokaryotes They can make their own proteins as they have ribosomes too!

Genes in Prokaryote Cells Flagellum Bacteria have no membrane- bound organelles. Cellular reactions occur on the inner surface of the cell membrane or in the cytoplasm. Bacterial DNA is found in: One, large circular chromosome. Several small chromosomal structures called plasmids. Cytoplasm (no nucleus) Cell membrane Single, circular chromosome Ribosomes Plasmids Cell wall

Plasmid DNA Bacteria have small accessory chromosomes called plasmids. Plasmids replicate independently of the main chromosome. Some conjugative plasmids can be exchanged with other bacteria in a process called conjugation. Via conjugation, plasmids can transfer antibiotic resistance to other bacteria. Recipient bacterium Sex pilus conducts the plasmid to the recipient bacterium Plasmid of the conjugative type A plasmid about to pass one strand of the DNA into the sex pilus Plasmid of the non-conjugative type Donor bacterium

How are genes arranged on a molecule of DNA Genes are sections of DNA. They code for a particular polypeptide Each DNA molecule has many genes along its length Each gene occupies a specific position on the chromosome = gene locus A lot of the DNA in eukaryotes does not code for amino acids. It is called non coding DNA

Coding and non coding DNA In eukaryotes much of the DNA in the nucleus does not code for polypeptides Non-coding DNA exists between genes. This can be repeating sequences as if the DNA is stuttering Exons are codes for polypeptides, introns are non-coding DNA within the gene

Chromosomes A Chromosomes can be represented in different forms by using a variety of microscopes: A: Light microscope view of a chromosome from the salivary glands of the fly Simulium. Banding: groups of genes stained light and dark. Puffing: areas of transcription (mRNA production). B: Scanning electron microscope (SEM) view of sex chromosomes in the condensed state during a cell division. Individual chromatin fibers are visible. The smaller chromosome is the ‘Y’ while the larger one is the X. C: Transmission electron microscope (TEM) view of chromosomes lined up at the equator of a cell during the process of cell division. These chromosomes are also in the condensed state. B C

Chromosome States Interphase: Chromosomes are single-armed structures during their unwound state during interphase. Dividing cells: Chromosomes are double-armed structures, having replicated their DNA to form two chromatids in preparation for cell division. Interphase chromosome This chromosome would not be visible as a coiled up structure, but unwound as a region of dense chromatin in the nucleus (as in the TEM of the nucleus above) Replicated chromosome prepared for cell division Chromatin Chromatid Centromere

(double helix comprising genes) Chromosome Structure Histone proteins organize the DNA into tightly coiled structures (visible chromosomes) during cell division. Coiling into compact structures allows the chromatids to separate without tangling during cell division. Cell Replicated chromosome DNA molecule (double helix comprising genes) Individual atoms Chromatin: a complex of DNA and protein Histone proteins

Chromosome Features Chromosomes can be identified by noting: Centromere position Metacentric Submetacentric or Subterminal Acrocentric Chromosomes can be identified by noting: Banding patterns Position of the centromere Presence of satellites Length of the chromatids These features enable homologous pairs to be matched and therefore accurate karyotypes to be made. Banding pattern Chromosome length Satellite endings

Human Karyotype

Describe the nature of homologous chromosomes What does the prefix Homo mean? Where does each chromosome in a homologous pair come from? What does the word diploid mean? What is the diploid number for humans? Why are they called homologous? What can be different about homologous chromosomes? What does “Haploid mean” and what cells are involved?

Describe the nature of homologous chromosomes What does the prefix Homo mean? Where does each chromosome in a homologous pair come from? What does the word diploid mean? What is the diploid number for humans? Why are they called homologous? What can be different about homologous chromosomes? What does “Haploid mean” and what cells are involved? The same Mother (maternal) and father (paternal) Total number of chromosomes in a body cell 46 Same gene loci as each other Different forms of the gene on each chromosome (=alleles) HAPLOID = half the number of diploid = the number of chromosomes in a sperm/egg. Produced by meiosis

Explain what is meant by an allele Give the definition of an allele Give 2 examples of alleles of different genes What happens to the allele if the gene mutates?

Objective 4 Explain what is meant by an allele Give the definition of an allele Give 2 examples of alleles of different genes What happens to the allele if the gene mutates? Different form of the same gene Tall and dwarf in tomato plants (T t) It mutates into a different allele.

Homework Summary questions p207 MAHAC

Assessment How assessed (teacher, peer, self etc.)- hide from students

Extension activity A textbook stated that “The bacterial chromosome is found in the cytoplasm of the cell”. Evaluate this statement. - Ask students to compare the structure of prokaryotic cells with mitochondria and chloroplasts, identify similarities and suggest a theory.

Keywords Prokaryotic, eukaryotic, chromosomes, chromatid, centromere, homologous, diploid, gene, allele,