1 Genes in Eukaryote CellsEukaryotes 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

2 Genes Outside the Nucleus in Eukaryote CellsRibosome 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

3 Genes in Prokaryote CellsFlagellum 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

4 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

5 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

6 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

7 (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

8 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

9 Human Karyotypes Karyotypes display the chromosome contents of a cell, organized according to their number, size and type. Normal somatic human cells have a karyotype with 46 chromosomes (in 23 pairs) comprising: 22 pairs of autosomes. 1 pair of sex chromosomes. These determine the sex of an individual: XX = female XY = male 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Y X Sex chromosomes

10 Human Female KaryotypeEvery cell (except egg cells) in a normal human female has: 44 autosomes 2 sex chromosomes Human Female: 44 + XX Sex chromosomes: XX = female

11 Human Male Karyotype Human Male: 44 + XYEvery cell (except sperm cells) in a normal human male has: 44 autosomes 2 sex chromosomes Human Male: XY Sex chromosomes: XY = male

12 Chromosomes Contain GenesA single chromosome may contain hundreds of genes. Below are the locations of some known genes on human chromosomes: 1 El Rh AMY Fy 1270 13 RB 195 TYS 4 MN 465 X CBD HEMA 773 9 ABO NP 499 Chromosome: No. of genes:

13 Numbers of ChromosomesChromosome numbers vary considerably among organisms. The numbers may differ markedly even between closely related species: Organisms Chromosome No. human 46 chimpanzee 48 gorilla cattle 60 cat 38 goldfish 94 Drosophila 8 honey bee 32 or 16 Hydra 32 cabbage 18 beans 22 orange 18, 27 or 36 garden pea 14

14 Nucleotides The building blocks of nucleic acids (DNA and RNA) comprise the following components: a sugar (ribose or deoxyribose) a phosphate group a base (four types for each of DNA and RNA) Phosphate Base Adenine Sugar

15 Structure of NucleotidesThe chemical structure of nucleotides: Symbolic form Phosphate: Links neighboring sugars Base: Four types are possible in DNA: adenine, guanine, cytosine and thymine. RNA has the same except uracil replaces thymine. Sugar: One of two types possible: ribose in RNA and deoxyribose in DNA

16 Nucleic Acids What does DNA look like?It’s not difficult to isolate DNA from cells. The DNA extracted from a lot of cells can be made to form a whitish, glue-like material. DNA

17 Types of Nucleic Acid Nucleic acids are found in two forms: DNA and RNA DNA is found in the following places: Chromosomes in the nucleus of eukaryotes Chromosomes and plastids of prokaryotes Mitochondria Chloroplasts of plant cells RNA is found in the following forms: Transfer RNA: tRNA Messenger RNA: mRNA Ribosomal RNA: rRNA Genetic material of some viruses

18 DNA & RNA Compared Structural differences between DNA and RNA include:Strands Double Single Sugar Deoxyribose Ribose Bases Guanine Cytosine Thymine Uracil Adenine

19 Nucleotide Bases Purines Adenine • Double-ringed structures Guanine • Always pair up with pyrimidines The base component of nucleotides which comprise the genetic code. Base component of a nucleotide Pyrimidines Cytosine • Single-ringed structures Thymine • Always pair up with purines Uracil

20 DNA Structure Phosphates link neighboring nucleotides together to form one half of a double-stranded DNA molecule: Purine base (guanine) Pyrimidine base (thymine) Purine base (adenine) Pyrimidine base (cytosine) Sugar (deoxyribose) Hydrogen bonds Phosphate

21 DNA Molecule Purines join with pyrimidines in the DNA molecule by way of relatively weak hydrogen bonds with the bases forming cross-linkages. This leads to the formation of a double-stranded molecule of two opposing chains of nucleotides: The symbolic diagram shows DNA as a flat structure. The space-filling model shows how, in reality, the DNA molecule twists into a spiral structure. Symbolic representation Space-filling model Hydrogen bonds

22 The Genetic Code DNA codes for assembly of amino acids.The code is read in a sequence of three bases called: Triplets on DNA Codons on mRNA Anticodons on tRNA Each triplet codes for one amino acid, but more than one triplet may encode some amino acids (the code is said to be degenerate). There are a few triplet codes that make up the START and STOP sequences for polypeptide chain formation (denoted below in the mRNA form): START: AUG STOP: UAA, UAG, UGA

23 The Genetic Code AUG ACG GUA UUA CCC GAA GGC UAA START STOP START: AUGSTOP: UAA, UAG, UGA EXAMPLE: A mRNA strand coding for six amino acids with a start and stop sequence: AUG ACG GUA UUA CCC GAA GGC UAA START STOP

24 Decoding the Genetic CodeAmino Acid Codons No. Alanine GCU GCC GCA GCG 4 Arginine CGU CGC CGA CGG AGA AGG 6 Asparagine AAU AAC 2 Aspartic Acid GAU GAC Cysteine UGU UGC Glutamine CAA CAG Glutamic Acid GAA GAG Glycine GGU GGC GGA GGG Histidine CAU CAC Isoleucine AUU AUC AUA 3 Leucine UAA UUG CUU CUC CUA CUG Lysine AAA AAG Methionine AUG 1 Phenylalanine UUU UUC Proline CCU CCC CCA CCG Serine UCU UCC UCA UCG AGU AGC Threonine ACU ACC ACA ACG Tryptophan UGG Tyrosine UAU UAC Valine GUU GUC GUA GUG Two-base codons would not give enough combinations with the 4-base alphabet to code for the 20 amino acids commonly found in proteins (it would provide for only 16 amino acids). Many of the codons for a single amino acid differ only in the last base. This reduces the chance that point mutations will have any noticeable effect.

25 Genes and Proteins Functional protein Three nucleotide bases make up a triplet which codes for one amino acid. Groups of nucleotides make up a gene which codes for one polypeptide chain. Several genes may make up a transcription unit, which codes for a functional protein. Polypeptide chain Gene Triplet

26 A triplet codes for one amino acidGenes and Proteins This polypeptide chain forms one part of the functional protein. Functional protein This polypeptide chain forms the other part of the functional protein. Amino acids A triplet codes for one amino acid Polypeptide chain Protein synthesis: transcription and translation TAC on the template DNA strand Gene Transcription unit Three nucleotides make up a triplet DNA 3 ' 5 ' START Triplet STOP Nucleotide In models of nucleic acids, nucleotides are denoted by their base letter.

27 Cell Division Somatic cell production Male adult 2N Female adultMany mitosis divisions Male adult 2N Female adult Female embryo 2N Male embryo Meiosis A single set of chromosomes Egg 1N Sperm Several mitotic divisions Somatic cell production A double set of chromosomes Embryo 2N Many mitotic divisions Somatic cell production Adult 2N Gamete production Fertilization Zygote 2N

28 Mitosis in Onion Cells If the cells of a growing root tip are examined, a proportion of them are in mitosis. Cells in different stages of division can be seen, but the majority of the cells are in interphase. This reflects the large proportion of the cell cycle spent in interphase. Prophase Anaphase Late anaphase Telophase

29 Mitosis Interphase Cell enters mitosis Early Prophase Late ProphaseNuclear Membrane Interphase Centrosome, which later forms the spindle, is also replicated. DNA is replicated to form 2 chromatids Nucleolus Cell enters mitosis Early Prophase DNA continues condensing into chromosomes and the nuclear membrane begins to dissolve Late Prophase Chromosomes continue to coil up and appear as double-chromatids Metaphase The mitotic spindle is formed to organize the chromosomes. The spindle consists of fibers made of microtubules and proteins. Anaphase The chromosomes segregate, pulling the chromatids apart Two new nuclei form. The cell plate forms across the midline of the parent cell. This is where the new cell wall will form. Telophase Division of the cytoplasm (cytokinesis) is complete. The two daughter cells are now separate cells in their own right. Cytokinesis Late Anaphase

30 Mitosis Micrographs Cell division for somatic growth and repair.1. Interphase 2. Prophase 3. Metaphase 4. Anaphase 6. Telophase 5. Late Anaphase