Chapter 2. An Introduction to Genes and Genomes

Transcription

1 PowerPoint Lectures for Introduction to Biotechnology, Second Edition William J.Thieman and Michael A.Palladino Chapter 2 An Introduction to Genes and Genomes Lectures by Lara Dowland

2 Chapter Contents 2.1 A Review of Cell Structure 2.2 The Molecule of Life 2.3 Chromosome Structure, DNA Replication, and Genomes 2.4 RNA and Protein Synthesis 2.5 Mutations: Causes and Consequences

3 2.1 A Review of Cell Structure Plasma Membrane double-layer structure of lipids and proteins that surrounds the outer surface of cells Cytoplasm inner contents of a cell between the nucleus and plasma membrane Organelles structures in the cell that perform specific functions

4 2.1 A Review of Cell Structure Prokaryotic Cells (include bacteria) No nucleus and no organelles True bacteria Eubacteria

5 DNA from a lysed E. coli cell. plasmid

6 2.1 A Review of Cell Structure Eukaryotic cells (plant cells, animal cells) Have a nucleus and many organelles Organelles Nucleus Mitochondria Endoplasmic reticulum Golgi apparatus

7 2.1 A Review of Cell Structure

8 2.1 A Review of Cell Structure Comparison of Prokaryotic and Eukaryotic Cells

9 2.2 The Molecule of Life DNA Structure Building block of DNA is the nucleotide Each nucleotide is composed of Pentose (5-carbon) sugar called deoxyribose Phosphate molecule A nitrogenous base The nitrogenous bases are the interchangeable component of a nucleotide Each nucleotide contains one base Adenine (A), thymine (T), guanine (G) or cytosine (C)

10 2.2 The Molecule of Life

11 2.2 The Molecule of Life DNA Structure James Watson and Francis Crick revealed the definitive structure of DNA The Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid published in Nature on April 25, 1953

12 2.2 The Molecule of Life DNA Structure Nucleotides are joined together to form long strands of DNA and each DNA molecule consists of two strands that join together and wrap around each other to form a double helix Nucleotides in a strand are held together by phosphodiester bonds Each strand has a polarity a 5 end and a 3 end

13 2.2 The Molecule of Life DNA Structure The two strands of a DNA molecule are held together by hydrogen bonds Formed between complementary base pairs Adenine (A) pairs with thymine (T) Guanine (G) pairs with cytosine (C) The two strands are antiparallel because their polarity is reversed relative to each other

14 2.2 The Molecule of Life

15 2.3 Chromosome Structure, DNA Replication, and Genomes Chromosome Structure Chromosomes highly coiled and tightly condensed package of DNA and proteins Occurs only during DNA replication Chromatin strings of DNA and DNA-binding proteins called histones State of DNA inside the nucleus when the cell is NOT dividing

16 Eukaryotic chromosomes Karyotype analysis: to study chromosome number and structure

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19 The structure of chromosome Chromosome: the nucleic acid molecule Chromatin: (DNA + histone/nonhistone protein) nucleosome Beads on the string

20 2.3 Chromosome Structure, DNA Replication, and Genomes DNA Replication Cells divide by a process called mitosis Sex cells divide by a slightly different process called meiosis Mitosis One cell divides to form two daughter cells, each with an identical copy of the parent cell DNA In order to accomplish this, the DNA of the parent cell must be copied prior to mitosis

21 2.3 Chromosome Structure, DNA Replication, and Genomes Semiconservative Replication Replication occurs in such a manner that, after replication, each helix contains one original (parental) DNA strand and one newly synthesized DNA strand

22 2.3 Chromosome Structure, DNA Replication, and Genomes

23 2.3 Chromosome Structure, DNA Replication, and Genomes Steps in DNA Replication 1. Unwinding the DNA Helicase enzyme breaks the hydrogen bonds holding the two DNA strands together; unzips DNA DNA binding proteins hold the strands apart Separation of strands occurs in regions called origins of replication 2. Adding short segments of RNA Primase enzyme adds RNA primers RNA primers start the replication process

24 2.3 Chromosome Structure, DNA Replication, and Genomes Steps in DNA Replication 3. Copying the DNA DNA polymerase enzyme binds to the RNA primers Uses nucleotides to synthesize complementary strands of DNA Always works in one direction 5 to 3 direction

25 2.3 Chromosome Structure, DNA Replication, and Genomes

26 2.4 RNA and Protein Synthesis Transcription genes are copied (transcribed) from DNA code to RNA code Translation RNA code is read into a protein

27 2.4 RNA and Protein Synthesis

28 2.4 RNA and Protein Synthesis Transcription Occurs only in genes RNA polymerase unwinds DNA helix and copies one strand of DNA into RNA Binds to a promotor region Copies DNA in a 5 to 3 direction into RNA Uses nucleotides Adenine, uracil, guanine, and cytosine A-U, C-G

29 2.4 RNA and Protein Synthesis Transcription At end of gene, RNA polymerase encounters the termination sequence RNA polymerase and newly formed strand of RNA are released from DNA molecule RNA strand is called a messenger RNA (mrna) Multiple copies of mrna are transcribed from each gene during transcription

30 2.4 RNA and Protein Synthesis

31 2.4 RNA and Protein Synthesis mrna Processing Initial mrna produced is the primary transcript Immature and not fully functional A series of modifications before primary transcripts are ready for protein synthesis RNA splicing Polyadenylation Addition of a 5 cap

32 Fig RNA processing: RNA splicing Pre-mRNA 5 Cap 5 mrna Exon Intron Cap Exon Coding segment Intron 105 Exon Introns cut out and exons spliced together UTR 3 UTR Poly-A tail Poly-A tail

33 Fig RNA processing: addition of the 5 cap and poly-a tail Polyadenylation sign 5 Protein-coding segment G P P P AAUAAA AAA AAA 5 Cap 5 UTR Start codon Stop codon 3 UTR Poly-A tail 3 ( As) Ribosome recognition Protect from RNA-degrading enzyme RNA stability

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35 1. Initiation 2. Elongation 3. Termination Transcription 4. Posttranscriptional processing of RNAs 5. Splicing Prokaryotes: 1. transcription: cytoplasm 2. transcription and translation come together 3. No processing is needed Eukaryotes: 1. transcription: nucleus 2. The RNA transcripts are modified in the nucleus before export to the cytoplasm 3. Processing: to increase RNA stability mGTP, 3 -poly(a) tail

36 2.4 RNA and Protein Synthesis Basics of Gene Expression Control Micro RNA (mirna) regulate gene expression by silencing gene expression through blocking translation of mrna or by causing degradation of mrna

37 2.4 RNA and Protein Synthesis How Is mrna read? Genetic code universal language of genetics used by virtually all living organisms Works in three nucleotide units of mrna called codons Each codon codes for a single amino acid One amino acid may be coded for by more than one codon Start codon Stop codons

38 2.4 RNA and Protein Synthesis

39 2.4 RNA and Protein Synthesis Translation Occurs in the cytoplasm Function of each type of RNA mrna exact copy of the gene; carries the genetic code from nucleus to the cytoplasm rrna component of ribosomes, the organelles responsible for protein synthesis trna transports amino acids to ribosome

40 2.4 RNA and Protein Synthesis Translation 1. Initiation small ribosome subunit binds to 5 end of mrna Moves along the mrna until the start codon is found 2. Elongation trnas, carrying the correct amino acid, enter the ribosome, one at a time, as the mrna code is read 3. Termination ribosome encounters the stop codon Newly formed protein is released

41 2.4 RNA and Protein Synthesis 4^3=64 20 aa

42 2.4 RNA and Protein Synthesis Basics of Gene Expression Control Gene expression refers to the production of mrna by a cell All cells of an organism contain the same genome, so how and why are skin cells different from brain cells or liver cells? Because cells can regulate or control the genes they express

43 2.4 RNA and Protein Synthesis Basics of Gene Expression Control Gene regulation is how genes can be turned on and off in response to different signals

44 2.4 RNA and Protein Synthesis

45 2.4 RNA and Protein Synthesis Basics of Gene Expression Control Transcriptional regulation controlling the amount of mrna transcribed from a particular gene Certain sequences found in the promoter region TATA box and CAAT box RNA polymerase cannot bind to promotor region without presence of transcription factors Enhancer sequences bind to regulatory proteins called activators

46 Fig DNA Enhancer 1. Activators Distal control element DNA-bending protein Promoter TATA box 3. General transcription factors Gene 2. Group of mediator proteins RNA polymerase II RNA polymerase II Transcription initiation complex RNA synthesis

47 2.4 RNA and Protein Synthesis Basics of Gene Expression Control Bacteria use operons to regulate gene expression Organization of bacterial genes Clusters of several related genes located together and controlled by a single promotor Operator region within promotor Can use operons to regulate gene expression in response to their nutrient requirements lac operon

48 2.4 RNA and Protein Synthesis

49 2.5 Mutations: Causes and Consequences Mutation change in the nucleotide sequence of DNA Major cause of genetic diversity Can also be detrimental Types of Mutations Point mutations Silent mutations Missense mutations Nonsense mutations Frameshift mutations

50 2.5 Mutations: Causes and Consequences

51 2.5 Mutations: Causes and Consequences Gene mutations can be inherited or acquired Inherited mutations are those passed on to offspring through gametes Acquired mutations occur in the genome of somatic cells Are not passed along to offspring

52 2.5 Mutations: Causes and Consequences Mutations are a major cause of genetic diversity Human genomes are approximately 99.9% identical 0.1% differences in DNA between individuals, or one base out of every thousand Roughly 3 million differences between different individuals Most have no obvious effects; other mutations strongly influence cell functions, behavior, and susceptibility to genetic diseases