genetic information, variation and relationship between organisms

Question 1

gene definition

Answer 1

A section of DNA that contains the coding info for making polypeptide and functional RNA

Question 2

Genome definition

Answer 2

The genome of an organism is the whole of its hereditary information encoded in its DNA

Question 3

Exon definition

Answer 3

Coding sequence in DNA

Question 4

Intro def

Answer 4

non coding sequence in DNA

Question 5

What is a gene

Answer 5

sequences of DNA bases that codes for a polypeptide
sequence determines the sequence of A acids in the primary structure of protein

some also code for functional RNA
e.g.rRNA and tRNA

gene occupies fixed position on particular chromosome called gene locus

homologous chromosomes carry same gene- inherit 1 half of pair from each parent

Question 6

gene in eukaryotic vs prokaryotic DNA

Answer 6

Eukaryotic-
genes contains both exon and intron

exons code for specific sequence of A acid in primary structure

introns Don’t code for this

introns found in intergenic region between genes

prokaryotic-
genes dont contain intons

Question 7

Genome to Proteome

Answer 7

genome= all DNA in a cell
includes linear and circular

proteome= full range of proteins produced by cells

during Protein synthesise ( transcription and translation) triplets are used to code for primary structure

DNA triplets are transcribed to comp mRNA codons

Triplet- consists of 3 base pairs e.g. ACT

Question 8

features of genetic code

Answer 8

universal- same 3 bases code (DNA triplet/mRNA codon) for same A acid in all organisms

non overlapping- each base is only read once in triplet/codon

degenerate- more than 1 triplet/ codon codes for a single amino acids
e.g. only 20 A acids but 64 combinations of triplets

Question 9

mRNA vs tRNA

Answer 9

mRNA (messenger RNA)-
polymer of RNA nucleotides (ribose sugar and AUGC)
Single stranded
linear
shorter than DNA longet than/ more nucleotides than tRNA
copy of gene that takes info to the ribsomes to make proteins
has Codon
No H bonds

tRNA (transfer RNA)-
polymer of RNA nucleotides (ribose sugar and AUGC)
single strand
folded into cloverleaf shape
held by H bonds between comp base pairs
has A Acid binding site
Has Anticodon
brings specific A acid to ribsome during translation

Question 10

transcription

Answer 10

production of mRNA from DNA in nucleus

1.DNA helicase breaks H bonds between comp base pairs

2.only 1 strand of DNA acts as template

3. free RNA nucleotides align with comp base pairs (H bonds)
4. uracil not thymine comp base pair with adenine

5.RNA polymerase joins adjacent RNA nucleotides via phosophodiester bonds

Eukaryotes only:

6.pre-mRNA spliced to remove introns producing mRNA

Question 11

stop codons

Answer 11

stop translation

result in detachment of polypeptide chain from ribosome

end of polypeptide chain

Question 12

translation

Answer 12

production of polypeptide

1) mRNA attaches to ribsome

2) tRNA brings specific A acid to ribsome

3)tRNA anticodon binds to comp mRNA codon, 2 tRNA anticodons bind to 2 mRNA codon at any one time in ribsome (form 2 weak H bond)

4) A acids joined by peptide binds (condensation reaction)- catalyzed by ribsome

5)requires energy realses from ATP hydrolysis

6) tRNA is realised after A acids joined in polypeptide

7)ribsome moves along mRNA to form polypeptide- released into cytoplasm

First codon is a ‘start’ codon
eventually ribsome reached ‘stop’ codon and detaches from mRNA ensuring polypeptide produced had correct length

Question 13

genetic mutation

Answer 13

a random change in the base sequence of DNA, that results in the formation of a new allel

Question 14

how do mutations effect structure of protein

Answer 14

many change the primary structure (no and sequence of A acids) of polypeptide chain

this is because different codon code for different A acids

this may then change the position of bonds in the tertiary structure as r groups position has changed

may make it non functional or improve it

Question 15

different types of mutation

Answer 15

substitution

addition

deletion

Question 16

substitution mutation

Answer 16

typical only changes 1 base in DNA triplet

this changes corresponding mRNA base in codon

possible producing a diff amino acid (only 1 potentially changed in primary stucture)

however due to degenerate code not all subs result in any change as may code for same A acid

in this case no changes to P structure or T structure of Polypeptide (silent mutation)

if diff A acid does arise as result then p and T structures are changed ( as P effects where bonds form in T as it changes position of the R groups)which may make it unable to complete its job or may make it better adapted e.g. more or less comp enzyme

One other type of sub Is formation of stop codon

this signals ribsome to detach from mRNA (stop translation)

causing shorter chain- negative effects

Question 17

addition and deletion mutations

Answer 17

DNA nucleotides bases are added or deleted within sequence

Frame shift occurs- alteration of the base triplets and codon downstream from the mutation

changing codons, which changes A acid produced (possibly or some may remain same), frame shift change P structure and therefore T structure ( due to changing positions of R groups changing bond positions)

Question 18

mutagenic agents

Answer 18

increase the rate if gene mutations e.g.

1) high energy radiation- can directly damage the DNA molecules (e.g. x rays)

2)carcinogens- DNA reactive chemicals that can directly and indirectly damage DNA e.g. nitrous acid can convert cytosine in DNA to uracil

3)biological agents- e.g. viruses that can insert there genetic material in host cells genome

Question 19

Meiosis

Answer 19

produces genetically different daughter cells e.g. gamete

prior: interphase + semi Conservative rep creating copy of each chromosome

Dipolid parent cells (2n) contaun pair of homologous chromosomes which carry same gene diff allels

meiosis 1 : independent seg of homologous chromosomes and crossing over cause genetic diversity

mainly homologous chromosomes are seperated producing 2 diploid cells

meiosis 2: sister chromatids are seperated

over all produces 4 haploid cells

new comb of allels and therefore produces geneti variation in pop

Question 20

Crossing over

Answer 20

Meiosis 1

rare event (10% of cells undergoing meiosis 1)

occurs during prophase 1

1) homologous pairs of chromosomes associate- bivalent
2)chiasmata form (non sis chromatids wrap around 1 another)
3)equal lengths of non sis chromatids exchanges
4)produces new comb of allels

Question 21

independent segregation

Answer 21

Meiosis 1

biggest source of genetic variation

happens in Metaphase 1

random assortment or shuffling of homologous chromosomes

during anaphase 1 each pair of H chromosomes is seperated and maternal and paternal chromosomes move to opp poles (2n > n)

daughter cells produced will randomly contain maternal or paternal half of H pair which determined by random assortment

number of possibilities = 2^n where n= no of pairs of homologous chromosomes

e.g. humans 23 pairs so 2^23 = 8388608 random assortments of chromosomes

this is why random fusion of gamete is major source of genetic variation

Question 22

non disjuction mutations

Answer 22

First or second division

when sister chromatids or homologous chromosomes don’t split

in both cases daughter cells produce extra chromosome (n + 1) or lack of chromosome (n-1)

cause of downsyndrome

Question 23

Role of phenotypic variation in natural selection

Answer 23

larger pop = more genetically diverse

key points:
Random mutations can produce new allels of a gene

the creates variation in phenotype

selection pressure

phenotype providing advantage more likely to survive and reproduce

these organisms pass on their allels

advantageous allel frequency increases over many gens

Question 24

2 diff types of selection

Answer 24

stabilizing and directional

Question 25

Stabilizing selection

Answer 25

occurs in all pops in stable enviro

selection pressure at both ends of distribution (selected against)

one with extreme phenotype do not survive

favour average more common phenotype (Selected for)

more likely to survive reproduce and pass on allels

allel frequency increases

reduces variability

therefore reduces opportunity for evolutionary change

Question 26

directional selection

Answer 26

average pop rep the optimum phenotype exiting environmental conditions

environment change may produce new selection pressure the favors extreme phenotype

more likely to survive and reproduce

pass on allels for extreme phenotype

allel frequency increases over many gen