Proposition 6: Information Encoded in Genes Regulates Protein Synthesis

Week 1: Science and the Cellular Basis of Life Proposition 1. Science is a Powerful Way of Understanding the Living World Proposition 2. Living things Possess Unique Characteristics Proposition 3. A Central Characteristic of Living Things is Cellular Structure Week 2: Genetic Variation Proposition 4: As a Result of their Cellular Structure, Living Things Vary in Physical Traits Proposition 5: Some of the Variation in Living Things is Encoded in Genes Proposition 6: Information Encoded in Genes Regulates Protein Synthesis Week 3: Evolution, Natural Selection and Speciation Proposition 7: By Virtue of their Genetic Traits, Some Living Things are Better Adapted to their Environment than Others Proposition 8: Living Things that are Better Adapted to their Environment Tend to survive and Leave more offspring Proposition 9: Living Things that Leave More Offspring Become More Frequent over Time Proposition 10: Over the Course of Many Years Living Things Change as Their Environment Changes

Chromosomes All organisms pass DNA to offspring when they reproduce In cells, each DNA molecule is organized as a chromosome chromosome –Structure consisting of DNA and associated proteins –Carries part or all of a cell’s genetic information –Eukaryotic cells have a number of chromosomes

Chromosome Duplication During most of a cell’s life, each of its chromosomes consists of one DNA molecule As it prepares to divide, the cell duplicates its chromosomes, so both offspring get a full set After chromosomes are duplicated, each consists of two DNA molecules (sister chromatids) attached to each other at a centromere

Key Terms sister chromatid –One of two attached members of a duplicated eukaryotic chromosome centromere –Constricted region in a eukaryotic chromosome where sister chromatids are attached

Sister Chromatids A duplicated chromosome consists of two long, tangled filaments (sister chromatids) bunched into an X shape

Chromosome Structure DNA in a nucleus is divided into chromosomes At its most condensed, a duplicated chromosome is packed tightly into an X shape A chromosome unravels as a single fiber – a hollow cylinder formed by coiled coils The coiled coils consist of a long molecule of DNA and associated proteins The DNA molecule wraps around a core of histone proteins, forming “beads” called nucleosomes The DNA molecule has two strands twisted in a double helix

Chromosome Number Eukaryotic DNA is divided among a number of chromosomes that differ in length and shape The sum of all chromosomes in a cell of a given type is the chromosome number Diploid cells have two of each type of chromosome Each species has a characteristic chromosome number

Human Chromosome Number Human body cells have 46 chromosomes (chromosome number 46) Human body cells have two of each type of chromosome (23 pairs) so the chromosome number is diploid (2n) Each pair of chromosomes has two versions, one maternal and one paternal

Types of Chromosomes Members of a pair of sex chromosomes differ among males and females – the differences determine an individual’s sex All others chromosomes are autosomes, which are the same in both females and males Autosomes of a pair have the same length, shape, and centromere location, and carry the same genes

Key Terms Chromosomes –DNA of a eukaryotic cell is divided among a characteristic number of chromosomes that differ in length and shape –Sex chromosomes determine an individual’s gender –Proteins associated with eukaryotic DNA help organize chromosomes so they can pack into a nucleus

Discovery of DNA Structure James Watson and Francis Crick’s discovery of DNA’s structure was based on many years of research by other scientists

DNA’s Building Blocks: Nucleotides A DNA nucleotide has a five-carbon sugar, three phosphate groups, and one of four nitrogen- containing bases How the four nucleotides — adenine (A), guanine (G), thymine (T), and cytosine (C) — are arranged in DNA was a puzzle that took over 50 years to solve

Chargaff’s Discovery 1950s: Erwin Chargaff made two discoveries: –Chargaff’s first rule: A = T and G = C (amounts of thymine and adenine in all DNA are the same, as are amounts of cytosine and guanine) –Chargaff’s second rule: Proportions of adenine and guanine differ among the DNA of different species

Structure of DNA carbon of a sugar is joined by a phosphate group to carbon of next sugar, forming 2 sugar- phosphate backbones running in opposite directions Inside are paired bases: A to T, and G to C

DNA’s Base-Pair Sequence The order in which one base pair follows the next varies tremendously among species (Chargaff’s second rule) Variations in base sequence are the source of life’s diversity

Key Concepts Structure of DNA –A DNA molecule consists of two long chains of nucleotides coiled into a double helix –Four kinds of nucleotides make up the chains: adenine, thymine, guanine, and cytosine –The order of these bases in DNA differs among individuals and among species

8.6 DNA Replication and Repair The order of nucleotide bases in a strand of DNA – the DNA sequence – is genetic information Descendant cells must get an exact copy of DNA When the cell reproduces, it must contain two sets of chromosomes: one for each of its future offspring DNA duplicates itself by DNA replication

DNA Replication The order of nucleotide bases in a strand of DNA – the DNA sequence – is genetic information –Descendant cells must get an exact copy of DNA When the cell reproduces, it must contain two sets of chromosomes: one for each of its future offspring –DNA duplicates itself by DNA replication

Semiconservative Replication of DNA A parental DNA strand serves as a template for assembly of a new strand of DNA The two parental DNA strands stay intact, and a new strand is assembled on each of the parental (old) strands Each new DNA molecule that forms consists of one old strand and one new strand

Summary of DNA Replication DNA Replication –Before a cell divides, it copies its DNA so that each of its descendants gets a full complement of hereditary information –Newly forming DNA is monitored for errors, most of which are corrected –Uncorrected errors may be perpetuated as mutations

Key Terms DNA Replication –Before a cell divides, it copies its DNA so that each of its descendants gets a full complement of hereditary information –Newly forming DNA is monitored for errors, most of which are corrected –Uncorrected errors may be perpetuated as mutations

Genes and DNA DNA contains all of the instructions for building a new individual The linear order or sequence of the four bases (A, T, G, C) in the DNA strand is the genetic information, which occurs in subsets called genes gene –Part of a DNA base sequence –Specifies an RNA or protein product

Converting a Gene to RNA: Transcription Transcription converts information in a gene to RNA Enzymes use the nucleotide sequence of a gene as a template to synthesize a strand of RNA (ribonucleic acid) transcription –Process by which an RNA is assembled from nucleotides using the base sequence of a gene as a template

RNA RNA is a single-stranded chain of four kinds of nucleotides Like DNA, a RNA nucleotide has a phosphate group, a sugar, and one of four bases, but RNA is slightly different: –The sugar in RNA is ribose, not deoxyribose –RNA uses the base uracil instead of thymine

Converting RNA to Protein: Translation Translation converts information in an mRNA to protein mRNA carries a protein-building message encoded in the sequence of sets of three nucleotide bases mRNA is decoded (translated) into a sequence of amino acids, resulting in a polypeptide chain that folds into a protein translation –Process by which a polypeptide chain is assembled from amino acids in the order specified by an mRNA

Gene Expression Transcription and translation are part of gene expression, a process by which information encoded by a gene is converted into a structural or functional part of a cell or a body gene expression –Process by which the information in a gene becomes converted to an RNA or protein product

DNA to RNA to Protein –The sequence of amino acids in a polypeptide chain corresponds to a sequence of nucleotide bases in DNA called a gene –The conversion of information in DNA to protein occurs in two steps: transcription and translation

Transcription During transcription, DNA acts as a template upon which a strand of RNA (transcript) is assembled from RNA nucleotides Each new RNA is complementary in sequence to the DNA template: G pairs with C; A pairs with U (uracil) RNA polymerase adds nucleotides to the end of a growing transcript

Translation –Messenger RNA (mRNA) carries DNA’s protein-building instructions –Its nucleotide sequence is read three bases at a time –Two other types of RNA interact with mRNA during translation of that code

Codons and the Genetic Code The protein-building information in mRNA consists of a sequence of three mRNA bases (codon); each designates a particular amino acid Example: AUG codes for the amino acid methionine (met), and UGG codes for tryptophan (trp) The four bases A, C, G, and U can be combined into 64 different codons, which constitute the genetic code

Codons and the Genetic Code codon –In mRNA, a nucleotide base triplet that codes for an amino acid genetic code –Complete set of sixty- four mRNA codons

Codons and Amino Acids There are only twenty kinds of amino acids found in proteins, so some amino acids are specified by more than one codon The order of mRNA codons determines the order of amino acids in the polypeptide that will be translated from it

Mutated Genes and their Products If a mutation changes the genetic instructions encoded in the DNA, an altered gene product may result Example: Hemoglobin consists of four polypeptides (globins) folded around a heme (iron-containing cofactor) –Various defects in the polypeptides can cause sickle-cell anemia

What Happens When a Single Base in DNA is Substituted by Another Base? In base-pair-substitution, a nucleotide and its partner in DNA are replaced by a different base pair Sickle-cell anemia results from a substitution of valine for glutamic acid base-pair substitution Type of mutation in which a single base-pair changes

Sickle Cell Anemia Substitution of valine for glutamic acid causes HbS protein to clump Normally round red blood cells are distorted into sickle shapes

Down Syndrome Mutations –Small-scale, permanent changes in the nucleotide sequence of DNA may result from replication errors –Such mutations can change a gene’s product such as the hemoglobin molecule in sickle-cell anemia –Other mutations can result in the deletion or addition of an entire chromosome –In Trisomy 21 (Mongolism, Down Syndrome) the individual has an extra chromosome (47 instead of 46)