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)