proposition 6: information encoded in genes regulates protein synthesis
TRANSCRIPT
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
• 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)