An Introduction to Genetics. 9.1 An Introduction to Genetics. An Introduction to Genetics. An Introduction to Genetics. DNA Deoxyribonucleic acid

Transcription

1 An Introduction to Genetics 9.1 An Introduction to Genetics DNA Deoxyribonucleic acid Information blueprint for life Reproduction, development, and everyday functioning of living things Only 2% coding 98% function unknown Before human genome project Believed 100,000 genes Now known only 20,000 Copyright 2009 Pearson Education, Inc., publishing as Benjamin Cummings. An Introduction to Genetics An Introduction to Genetics DNA structure first described by James Watson and Francis Crick based their work on X-ray crystallography Rosalind Franklin and Maurice Wilkins 1953 published Double Helix Noble Prize 1

2 DNA DNA DNA Codes for proteins Enzymes Biocatalysts Metabolism Structure Cytoskeleton Packaging Receptors 4 letters of the DNA alphabet adenine (A) - pyrimidine thymine (T) - purine guanine (G) - pyrimidine cytosine (C) - purine Thymine (T) Pyrimidines Cytosine (C) Adenine (A) Purines Guanine (G) DNA Structure Double helix Twisted ladder Sugar-phosphate backbone Side rails of ladder Contains Deoxyribose sugar Phosphate group Nitrogenous base Rungs of the ladder Nucleotide Base + Sugar + Phosphate Nucleotides Monomer Nucleic acid DNA Structure Polymer Built with dehydration synthesis Broken apart with hydrolysis 2

3 Phosphate group Nitrogenous base Sugar DNA DNA Structure Sugar phosphate backbone Phosphate group Nitrogenous base (A, G, C, or T) DNA Specific pairing of bases A - T (pyrimidine - purine) G - C (pyrimidine - purine) Thymine (T) Sugar (deoxyribose) DNA nucleo@de DNA polynucleo@de DNA DNA Base pair Hydrogen bond Sequence of bases Codes for proteins One series of bases codes for one protein, while another codes a different protein. Each series of protein-specifying bases is known as a gene. Ribbon model Par@al chemical structure Computer model One gene equals one protein Other regions of DNA are regulatory Control what proteins are being made when 3

4 Protein Synthesis Protein Synthesis Protein synthesis Begins with transcription DNA template is transcribed copied into messenger RNA (mrna)» Single stranded nucleic acid» Uses ribose sugar instead of deoxyribose sugar» Uses uracil instead of thymine Occurs in the nucleus mrna Leaves nucleus Travels to a ribosome Ribosome made of ribosomal RNA (rrna) and structural proteins Ribosome matches sequence in mrna with transfer RNA (trna) and the amino acids they carry These amino acids are linked together into polypeptides The precursor to protein Protein Synthesis Genome DNA mrna nucleus cytosol 1.The information contained in a length of DNA is transcribed onto a length of messenger RNA (mrna). 2.The mrna then exits the cell s nucleus and goes to a structure in the cell s cytoplasm called a ribosome. amino acids Genome The full set of genes an organism contains Includes regulatory regions as well Therefore - all the DNA in a cell Each cell in an organism contains a complete copy of that organism s genome Except germ cells - sperm and eggs ribosome protein 3.Here, the mrna sequence is read, and a string of amino acids is put together in the order specified by the sequence. The result is a protein. Figure 9.3 4

5 Genome Before cells divide, their genome must first be copied and the resulting copies apportioned evenly into what will become two daughter cells. 9.2 An Introduction to Cell Division Copyright 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Cell Division Cell Division Cell division takes place because: Cells die and need to be replaced. Cells can only grow so large before they become dysfunctional. There are times in which an organism needs quantities of new cells above replacement level. Cell division includes: The duplication of DNA (replication) The apportioning of the copied DNA into two quantities in a parent cell (mitosis) The physical splitting of this parent cell into two daughter cells (cytokinesis) 5

6 Cell Division DNA Replication cell nucleus 1.Replication DNA is duplicated. 2.Mitosis The two quantities of DNA are moved to opposite sides of the parent cell. 3.Cytokinesis The parent cell splits into two daughter cells. In DNA replication, the two strands of the double helix unwind, after which each single strand serves as a template for construction of a second, complementary strand of DNA. The result is a doubling of the original quantity of DNA. Figure 9.4 DNA Replication 1.Original DNA molecule unwinds. 9.3 DNA is Packaged in Chromosomes 2.New DNA strands are synthesized from the two original strands. Figure 9.5 Copyright 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 6

7 Chromosomes Chromosomes and DNA Replication DNA comes packaged in units called chromosomes. Chromosomes are composed of DNA and its associated proteins a combined chemical complex called chromatin. Chromosomes exist in an unduplicated state until such time as DNA replicates, prior to cell division. DNA replication results in chromosomes that are in duplicated state, meaning one chromosome composed of two identical sister chromatids. Chromosomes and DNA Replication (a) DNA is packaged in units called chromosomes DNA wraps around protein to make chromatin. DNA (b) DNA replication at two levels Chromatin folds up to make chromosomes. chromatin DNA replication... duplicated chromosome cell... has this effect at the chromosomal level. unduplicated chromosome (not actual shape) Matched Pairs Homologous Chromosomes matched pairs of chromosomes one from each parent same sets of genes on them Not identical One of the sources of variation in populations duplicated chromosome sister chromatids Figure 9.6 7

8 Chromosomes Karyotype 46 chromosomes in humans Diploid number 23 homologous pairs Haploid number 22 autosomes 1 pair of sex chromosomes either a matched pair of X chromosomes (in females) or an X and a Y chromosome (in males). The two chromosomes above each number are a homologous pair (they are the same in size and function). A human male has one pair of non-homologous chromosomes, X and Y (females have XX). Figure 9.7 The Cell Cycle The Cell Cycle Cell division part of the cell cycle repeating pattern of growth, genetic replication, and cell division Two main phases Interphase Most of cell cycle Includes G1, S, and G2 G1 = growth S = synthesis» DNA replication G2 = growth in preparation of division Mitotic phase Cell division 8

9 The Cell Cycle The Cell Cycle Mitotic phase duplicated chromosomes separate Mitosis cell splits in two Cytokinesis 4.Mitotic Phase Mitosis Now the cell begins the process of apportioning its DNA into two opposite sides of itself. 3.Interphase G 2 In this gap 2 phase, DNA replication has been completed, and the cell continues with its normal functions, even as it prepares for mitosis and cytokinesis. G 2 M Cell cycle G 1 S 5.Mitotic Phase Cytokinesis With mitosis nearly complete, the cell begins the process of splitting into two daughter cells. Once this is finished, each of the two cells moves back into G 1 of interphase 1.Interphase G 1 In this gap 1 phase of the cell cycle, the cell is growing and carrying out its normal functions. 2.Interphase S In this synthesis phase, the cell is replicating its DNA (duplicating its chromosomes), in preparation for mitosis and cytokinesis. Figure 9.9 Mitosis and Cytokinesis 9.4 Mitosis and Cytokinesis Four stages in mitosis Prophase Metaphase Anaphase Telophase Copyright 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 9

10 Mitosis Mitosis Prophase Metaphase Chromosomes form as replicated chromatin material condenses Sister chromatids Nuclear membrane breaks down Mitotic spindle begins to form Chromosomes line up at metaphase plate Equator of the cell Centrioles migrate to poles of cell Spindle fibers attach to centromere of chromosomes Microtubules connecting a pair of centrioles chromosomes (each a pair of sister chromatids joined together) mitotic spindle metaphase plate spindle fibers (microtubules) Mitosis Mitosis Anaphase Telophase Sister chromatids separate As microtubules shorten and pull them apart at the centromere Pulled to centrioles at poles separating chromatids Chromosomes decondense Nuclear envelopes form around separated chromosomes sets Cleavage furrow begins to form cleavage furrow spindle fibers shortening 10

11 Mitosis Mitosis and Cytokinesis Mitosis and cytokinesis chromosomes (each a pair of sister chromatids joined together) mitosis pair of centrosomes nucleus mitotic spindle replicated, uncondensed DNA spindle fibers (microtubules) metaphase plate End of intephase Prophase mitosis begins Metaphase attachment and alignment DNA has already duplicated back in S phase. Centrosome has doubled. Chromosomes take shape; the two centrosomes begin to move toward the cellular poles, sprouting microtubules as they go. Microtubules attach to sister chromatids and align them at the metaphase plate. Figure 9.10 (1 of 2) Mitosis and Cytokinesis Mitosis and Cytokinesis spindle fibers shortening Anaphase separation separating chromatids Telophase and cytokinesis exit from mitosis cleavage furrow Completion of cytokinesis one cell becomes two Beginning of interphase After cell division sister chromatids now reside in separate daughter cells each sister chromatid now functioning as a fullfledged chromosome. Sister chromatids are moved to opposite poles in the cell, each chromatid now becoming a full-fledged chromosome. Chromosomes decondense; The cell membrane nuclear envelopes form around pinches together completely; membranes on the two separate complements of chromosomes. Cleavage either side fuse together, furrow begins to form. creating two cells. These two cells now enter the G 1 phase of interphase. Figure 9.10 (2 of 2) 11

12 Mitosis and Cytokinesis Cytokinesis in Animals Cytokinesis in animal cells a ring of protein filaments that tightens at the middle of a dividing cell. Figure 9.11 Variations in Cell Division 9.5 Variations in Cell Division Cytokinesis in plants Different from animal cells Have cell walls, so can t pinch off grows new cell walls and plasma membranes near the metaphase plate Yields two daughter cells. Copyright 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 12

13 Cytokinesis in Plants Binary Fission cell wall vesicles plasma membrane two daughter cells In Prokaryotes Double their genome single, circular chromosome two resulting chromosomes attached to different sites on the plasma membrane 1.Membrane-lined vesicles accumulate near the metaphase plate. The vesicles contain precursors to the cell wall. 2.Vesicles fuse together, forming a cell plate that grows toward the parent cell wall. 3.The newly formed plasma membrane and cell wall fuse with the parent plasma membrane and cell wall, forming two distinct daughter cells. Figure 9.12 Binary Fission Binary Fission in Bacteria Septum outgrowth of plasma membrane and cell wall, begins growing from opposite sides of the cell between the two chromosomes the two septum extensions join in the middle divide the one cell into two cell wall chromosome cell membrane parental bacterial cell two daughter cells 1.Bacterial cell starts with a single, circular chromosome attached to its plasma membrane. 2.The chromosome replicates and the daughter chromosomes attach to different sites on the plasma membrane. 3.The cell membrane and wall grow an extension between the attachment points of the two chromosomes. 4.The cell wall and membrane join together in the middle, resulting in two new cells. Figure

14 Cell Division in Bacteria binary fission 14