3.4 Genetic Information, Variation and Relationships between Organisms

Question 1

Define and outline a gene

Answer 1

• A section or base sequence of DNA located at a specific locus on a DNA molecule

• The base sequence of a gene carries coded genetic information that determines the amino acid sequence of a polypeptide and functional RNA

Question 2

Identify what causes adaptation

Answer 2

Genetic diversity being acted upon by natural selection

Question 3

How can variation be measured within a species

Answer 3

Using differences in the base sequence of DNA or in the amino acid sequence of proteins

Question 4

How can biodiversity be measured

Answer 4

• Species richness
• An index of diversity

Question 5

Outline Prokaryotic DNA

Answer 5

DNA molecules are short, circular and not associated with proteins

Question 6

Outline Eukaryotic DNA

Answer 6

• In the nucleus, DNA molecules are very long, linear and associated with proteins called histones
• Together a DNA molecule and it’s associated proteins form a chromosome

Question 7

Outline DNA in the mitochondria and chloroplasts of Eukaryotic cells

Answer 7

Like prokaryotic DNA, is short, circular and not associated with proteins

Question 8

Identify the differences between Prokaryotic and Eukaryotic DNA

Answer 8

• Eukaryotic DNA’s in the nucleus while prokaryotic DNA’s free in the cytoplasm

• Eukaryotic DNA’s linear while prokaryotic DNA’s circular

• Eukaryotic DNA’s longer the prokaryotic DNA

• Eukaryotes have exons and introns, prokaryotes only have exons

• Eukaryotic DNA’s wrapped around/associated with proteins called histones, prokaryotic DNA isn’t associated with proteins

Question 9

Define an exon

Answer 9

Sections of DNA that are always expressed in a protein

Question 10

Define an intron

Answer 10

Sections of DNA that don’t code for anything

Question 11

Define and outline an allele

Answer 11

• One of alternate forms of the same gene
• Each individual inherits one allele from each parent that are dominant or recessive and may be the same or different
• If different, there’s a difference in the base sequence so there’s a different amino acid sequence producing a different polypeptide

Question 12

Define a locus

Answer 12

The fixed position of a gene on a particular DNA molecule

Question 13

How many amino acids are there that regularly occur in proteins

Answer 13

20

Question 14

What are the four DNA bases

Answer 14

• Adenine
• Thymine
• Cytosine
• Guanine

Question 15

How are amino acids coded for

Answer 15

A sequence of three DNA bases called a triplet or codon that codes for a specific amino acid

Question 16

Identify the main features of the genetic code

Answer 16

• Degenerate; some amino acids are coded for by more than one codon

• Universal; the same codons code for the same amino acids in every organism (indirect evidence for evolution)

• Non-overlapping; each base is only read once in a codon

• Triplet; 3 DNA bases, triplet of bases code for one amino acid (codon)

Question 17

What’s a codon

Answer 17

A triplet of bases coding for an amino acid

Question 18

Identify three features of codons within the genetic code

Answer 18

• The start of the sequence is always methionine

• Three codons don’t code for any amino acids and act as stop codons signalling the end of a polypeptide chain

• Much Eukaryotic nuclear DNA doesn’t code for proteins (introns)

Question 19

Define and outline chromosomes

Answer 19

• Sister chromatids held together by the centromere
• Visible during cell division, but for the rest of the time are widely dispersed throughout the nucleus

Question 20

Outline haploid cells

Answer 20

• Cells with one set of chromosomes
• The result of meiosis
• All gametes (sex cells) are haploid

Question 21

Outline diploid cells

Answer 21

• Cells with homologous chromosomes
• Created through mitosis
• All somatic (body cells) are diploid

Question 22

Define the genome

Answer 22

The complete set of genes it genetic material present in a cell or organism

Question 23

Define the proteome

Answer 23

The full range of proteins that a cell is able to produce

Question 24

Outline the structure and function of messenger RNA (mRNA)

Answer 24

• Structure:
- Single strand, single gene long (size depends on
the length of the gene) with bases in triplets

• Function:
- Made during transcription, carries the genetic
code to the ribosome that is translated into a
polypeptide

Question 25

Outline the structure and function of transfer RNA (tRNA)

Answer 25

• Structure: - Single strand, folded into a clover leaf with hydrogen bonds between complimentary bases - Always the same size - Three free bases, with the anticodon at one end and the amino acid bonding site at the other • Function: - Carries amino acids to the ribosome during translation

Question 26

Identify the two processes is protein synthesis split into in order

Answer 26

• Transcription • Translation

Question 27

Define transcription, referencing the difference between prokaryotes and eukaryotes

Answer 27

The production of mRNA from DNA • In prokaryotes, transcription results directly in the production of mRNA from DNA • In Eukaryotes, transcription results in the production of pre-mRNA which is then spliced to form mRNA

Question 28

Describe the process of transcription in prokaryotes

Answer 28

• Hydrogen bonds between complimentary DNA strands are broken • One strand acts as the template strand • RNA nucleotides will align by complimentary base pairing, (uracil-adenine/cytosine-guanine) • RNA Polymerase joins adjacent nucleotides together • The DNA helix joins back together to form mRNA

Question 29

Describe the process of transcription in eukaryotes

Answer 29

• Hydrogen bonds between complimentary DNA strands are broken • One strand acts as the template strand • RNA nucleotides will align by complimentary base pairing, (uracil-adenine/cytosine-guanine) • RNA Polymerase joins adjacent nucleotides together • The DNA helix joins back together and pre-mRNA’s modified by splicing where pre-mRNA is placed to form mRNA

Question 30

Describe the process of splicing in eukaryotic transcription

Answer 30

• Introns are removed and exons are spliced back together in order • pre-mRNA is spliced to form mRNA

Question 31

Define translation

Answer 31

The production of polypeptides from the sequence of codons carried by mRNA

Question 32

Describe the process of translation

Answer 32

• mRNA associates with a ribosome • tRNA with a complimentary anticodon, binds to the first mRNA codon • The tRNA brings a specific amino acid, and the second tRNA brings a second specific amino acid and so on • Using ATP, the ribosome catalyses a peptide bond between amino acids • The first tRNA’s released from the ribosome, which moves along the mRNA to the next codon and the process repeats until it reaches a stop codon

Question 33

Outline the structure and function of a ribosome during translation

Answer 33

• Structure: Formed of protein and RNA and made up of a large and small subunit, that fit either side of the mRNA strand, they can be free in the cytoplasm or attached to the endoplasmic reticulum • Function: Site of protein synthesis via translation, the large subunit joins amino acids and the small subunit contains the mRNA binding site

Question 34

How many ribosomes can be found in a mammalian cell

Answer 34

Up to 10million

Question 35

Outline the role of ATP in translation

Answer 35

• Small and soluble, so it can be transported around the cell • ATP can’t pass out of the cell so cells always have an immediate supply of energy • Hydrolysis is a single reaction, so energy’s released quickly and it rapidly resynthesises • Low activation energy so it can release stored energy quickly • Stores and releases small amounts of energy at a time, so no energy’s wasted • Can transfer energy from one molecule to another by transferring one of its phosphate groups

Question 36

Why can’t DNA leave the nucleus

Answer 36

It’s too large and associated with proteins called histones

Question 37

Outline meiosis

Answer 37

• Meiosis is a form of reduction division that produces daughter cells that are genetically different from each other • Two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell • Genetically different daughter cells results from the independent segregation of homologous chromosomes • Crossing over between homologous chromosomes results in further genetic variation among daughter cells • Independent segregation of chromosomes and crossing over of homologous chromosomes increase variation in meiosis because of the new combinations of alleles in gametes. • New alleles are only caused by mutations that can only be passed on in meiosis, not mitosis

Question 38

Identify gametes in different eukaryotes

Answer 38

• Sperm and ova in animals • Pollen and ova in plants

Question 39

Define reduction division

Answer 39

The chromosome number’s reduced and haploid cells are formed

Question 40

Describe the process of meiosis

Answer 40

Prophase I • Replicated chromosomes condense to form chromatin and become visible • Crossing over can occur between homologous chromosomes Metaphase I • Homologous chromosomes line up randomly in pairs on the equator of the cell through independent segregation Anaphase I • Spindle fibres, made by centrioles, attach to the centromeres and chromosomes are pulled to opposite ends of the cell. Homologous chromosomes are now separated Telophase I • Chromosomes have gathered at either end of the cell and cytokinesis occurs MEIOSIS II Prophase II • Two separate cells Metaphase II • Chromosomes mine up on the equator of the cell, one on top of eachother Anaphase II • Spindle fibres attach to the centromeres and pull in the opposite directions. Centromeres break, sister chromatids are separated and pulled to opposite poles of the cell Telophase II • Cytokinesis occurs and four non-identical haploid gametes (sex cells) are produced

Question 41

Describe the process of crossing over during Prophase I of meiosis

Answer 41

• Crossing over can occur between homologous chromosomes, but doesn’t always happen • Homologous chromosomes ‘cross over’ each other to form a chiasma. This produces new combinations of alleles, increasing variation in meiosis

Question 42

Define homologous chromosomes

Answer 42

• A pair of chromosomes with the same genes at the same loci, but are not identical. • For example, one of the homologous chromosomes may have the gene for eye colour at ‘point A’ for blue eyes while the other homologous chromosome will also have the gene for eye colour at ‘point A’ but will have an allele for brown eyes

Question 43

Outline cytokinesis

Answer 43

• The separation of the parent cell into two genetically identical daughter cells once a new nucleus has completely reformed at each like if the parent cell after telophase • In animals, a ‘cleavage furrow’ forms and separates the daughter cells but in plants a ‘cell plate’ forms at the metaphase plate. Once it reaches the walls of the parent cell, new cell walls are produced, separating the new daughter cells

Question 44

Define and outline mutations

Answer 44

• A random change in the base sequence of chromosomes. They can arise spontaneously during DNA replication (mitosis/meiosis) • Mutations often have no effect but can be harmful or beneficial. Types of mutation can include base deletion and base substitution mutations. Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis • Mutagenic agents can increase the rate of gene mutation

Question 45

Outline base substitution mutations

Answer 45

• When one nucleotide is replaced by a different nucleotide E.g. TAG becomes TCG • There are three possible consequences ; 1. Nonsense mutations: The codon could be changed to a stop codon 2. Missense mutations: The codon could code for a different amino acid 3. Silent mutation: The codon could code for the same amino acid. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids

Question 46

Outline base deletion mutations

Answer 46

When one nucleotide is removed from a codon E.g. TAG becomes T G. • This causes a frame shift of subsequent codons, because genetic code is triplet and non overlapping all following codons will be different

Question 47

Outline chromosome mutations

Answer 47

• There are two categories caused by errors during meiosis: • Polyploidy is when an organism has 3 or more sets of chromosomes instead of the usual 2 (diploid). This usually happens in plants and is catastrophically damaging in humans • Chromosome non-disjunction is when chromosomes fail to separate during meiosis, gametes may end up with either no copy or an extra copy of a chromosome

Question 48

Outline and describe the process of chromosome non-disjunction

Answer 48

• Chromosome non-disjunction is when chromosomes fail to separate during meiosis, gametes may end up with either no copy or an extra copy of a chromosome • Homologous chromosomes don’t separate in meiosis I or sister chromatids don’t separate in meiosis II leading to gametes with an additional chromosome or one fewer chromosome (aneuploidy) this can cause: • Live births with triosmy 13/18/21, but only 21 (Down syndrome) is survivable to adulthood • Anaeuploidy

Question 49

Outline Aneuploidy

Answer 49

• There are multiple types, Autosomal refers to body chromosomes (not X/Y sex chromosomes) • Autosomal Aneuploidy is almost always fatal or causes a miscarriage • Sex Chromosome Aneuploidy is survivable to adulthood (Common types include: XXY, XYY, XXX, XO) • Mosaic Aneuploidy is where some cells have an additional chromosome • Non-mosaic Aneuploidy is where all cells have an additional chromosome

Question 50

Define genetic diversity

Answer 50

• The number of different alleles of genes in a population • Genetic diversity is a factor enabling natural selection to occur

Question 51

Identify the types of selection

Answer 51

• Directional selection • Stabilising selection • Disruptive selection

Question 52

Describe directional selection and provide and example

Answer 52

• Selection for an extreme of a phenotype from the mean; anything that increases or decreases is directional selection • E.g. selection for shorter fur in a warmer climate/ antibiotic resistance in bacteria

Question 53

Describe stabilising selection and provide and example

Answer 53

• Selection for the mean phenotype; the frequency of the mean phenotype increases over time • E.g. average body weight of babies when they’re born (too light/heavy leads to complications)

Question 54

Describe disruptive selection and provide and example

Answer 54

• Selection for both extremes of a phenotype at the same time. This requires isolation between the population and leads to speciation. • E.g. birds on an island with different beak sizes (thin beaks can eat insects, medium can eat fruit and thick can eat nuts/seeds). If there’s a drought, there’s no fresh fruit so only thick and thin beaks can survive. Eventually, they’ll be so genetically different they can’t interbreed, therefore there’s a new species

Question 55

Outline natural selection and its principles

Answer 55

• Random mutation can result in new alleles of a gene • Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success • The advantageous allele is inherited by members of the next generation • As a result, over many generations, the new allele increases in frequency in the population (increased allele frequency) • Results in species that are better adapted to their environment. These adaptations may be anatomical, physiological or behavioural

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Question 70

Outline classification of species

Answer 70

• A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships. • It uses a hierarchy in which smaller groups are placed in larger groups, with no overlap between groups. Each group is called a taxon (pl. taxa) • One hierarchy comprises the taxa: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species • Each species is universally identified by a binomial nam(e/ing system) made up by the genus and species e.g. Homo sapiens • The binomial name’s always written in *italics* (underlined in handwriting). The genus always starts with a capital letter and the species with a lower case letter

Question 71

Define a hierarchy

Answer 71

Smaller groups are placed in larger groups, with no overlap between groups