02 Lecture Presentation (Bio20-1, Mapua Institute of Technology)

Copyright © 2009 Pearson Education, Inc.

PowerPoint Lectures forIntroduction to Biotechnology, Second Edition

William J.Thieman and Michael A.Palladino

Lectures by Lara Dowland

Chapter 2An Introduction to Genes and Genomes


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


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


• Prokaryotic Cells (include bacteria)– No nucleus and no organelles




• Eukaryotic cells (plant cells, animal cells)– Have a nucleus and many organelles– Organelles

• Nucleus

• Mitochondria

• Endoplasmic reticulum

• Golgi apparatus



• Comparison of Prokaryotic and Eukaryotic Cells


2.2 The Molecule of Life

• Evidence that DNA is the Inherited Genetic Material– 1869 Friedrich Miescher: “nuclein”

• Could not be broken down by proteases• Had acidic properties: “nucleic acids”

– 1928 Frederick Griffith• Two strains of Streptococcus pneumoniae

– Virulent smooth strain (S cells) and harmless rough strain (R cells)

• Demonstrated transformation – the uptake of DNA by bacterial cells



• Evidence that DNA Is the Inherited Genetic Material– 1944 Oswald Avery, Colin MacLeod, and Maclyn

McCarty• Purified DNA from large batches of Streptococcus

pneumoniae

• Experiment proved that DNA was the transforming factor in the Griffith experiments



• 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)



• 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



• 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



• 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


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



• Most human cells have two sets (pairs) of 23 chromosomes, or 46 chromosomes total– Called homologous pairs– Autosomes – chromosomes 1-22– Sex chromosomes – chromosome pair # 23

• X and Y chromosomes

• Gametes (sex cells) contain a single set of 23 chromosomes (haploid number, n)



• Chromosome consists of two thin, rodlike structures of DNA called sister chromatids– Exact replicas of each other copied during DNA

replication– During cell division, each sister chromatid is

separated



• 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



• 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



Steps in DNA Replication1. 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



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


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



• Transcription– Occurs only in genes– RNA polymerase unwinds DNA helix and copies one

strand of DNA into RNA• Binds to a promoter region

• Copies DNA in a 5’ to 3’ direction into RNA

• Uses nucleotides

– Adenine, uracil, guanine, and cytosine

– A-U, C-G



• 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



• 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



• 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



• 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



Translation1. 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



• 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



• Basics of Gene Expression Control– Gene regulation is how genes can be turned on and off

in response to different signals



• Basics of Gene Expression Control– Transcriptional regulation – controlling the amount of

mRNA transcribed from a particular gene• Certain sequences found in the promotor 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



• 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



• 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


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



• 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



• 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

02 Lecture Presentation (Bio20-1, Mapua Institute of Technology)

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