genes, protein synthesis, meosis, mutations

Question 1

Compare and contrast the similarities of DNA in eukaryotic cells with DNA in prokaryotic cells

Answer 1

● Nucleotide structure is identical - deoxyribose attached to phosphate and a base
● Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds
● DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes
○ Short, circular, not associated with proteins

Question 2

Compare and contrast the differences of DNA in eukaryotic cells with DNA in prokaryotic cells

Answer 2

● Eukaryotic DNA is longer
● Eukaryotic DNA is linear, prokaryotic DNA is circular
● Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not
● Eukaryotic DNA contain introns, prokaryotic DNA does not

Question 3

What is a chromosome?

Answer 3

● Long, linear DNA + its associated histone proteins
● In the nucleus of eukaryotic cells

Question 4

What is a gene?

Answer 4

A section of DNA that codes for the sequence of amino acids in a polypeptide and functional RNA (e.g. ribosomal RNA/tRNA)

Question 5

What is a locus?

Answer 5

the location of the gene on the strand of DNA/chromosome

Question 6

Describe the nature of the genetic code

Answer 6

-Triplet code: A sequence of 3 DNA bases, called a triplet, codes for a specific amino acid
-Universal: The same base triplets code for the same amino acids in all organisms
-Non-overlapping: Each base is part of only one triplet so each triplet is read as a discrete unit
-Degenerate: An amino acid can be coded for by more than one base triplet
-each triplet is always read in the same direction (5’ to 3’)
-each gene always begins with the same triplet

Question 7

What are ‘non-coding base sequences’ and where are they found?

Answer 7

• DNA that does not code for amino acid sequences / polypeptides:
  1. Between genes - eg. non-coding multiple repeats
  2. Within genes - introns
  In eukaryotes, much of the nuclear DNA does not code for polypeptides

Question 8

true or false “Bacteria / prokaryotic cells have
chromosomes.”

Answer 8

false
Chromosomes are found in eukaryotic cells. Bacteria have small, circular DNA not associated with histones.

Question 8

What are introns and exons?

Answer 8

-Exon: Base sequence of a gene coding for amino acid sequences (in a polypeptide)
-Intron: Base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells

Question 9

true or false “Prokaryotic DNA is single stranded, and contains uracil instead of thymine.

Answer 9

false
This describes RNA. Prokaryotic DNA is double stranded and made
of exactly the same nucleotides as eukaryotic DNA (with A, T, C, G).

Question 10

true or false
“Triplets produce amino acids.”

Answer 10

Triplets code for amino acids, but don’t directly produce them.

Question 11

Define ‘genome’

Answer 11

The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)

Question 12

define proteome

Answer 12

The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Question 13

Describe the two stages of protein synthesis

Answer 13

-Transcription: Production of messenger RNA (mRNA) from DNA, in the nucleus
-Translation: Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

Question 14

Compare and contrast the structure of tRNA and mRNA

Answer 14

-similarities:
● Both single polynucleotide strand
-differences:
● tRNA is folded into a ‘clover leaf shape’, whereas
mRNA is linear / straight
● tRNA has hydrogen bonds between paired bases,
mRNA doesn’t
● tRNA is a shorter, fixed length, whereas mRNA is a
longer, variable length (more nucleotides)
● tRNA has an anticodon, mRNA has codons
● tRNA has an amino acid binding site, mRNA doesn’t

Question 15

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 15

1. Hydrogen bonds between DNA bases break
2. Only one DNA strand acts as a template
3. Free RNA nucleotides align next to their complementary bases on the template strand
   ○ In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
4. RNA polymerase joins adjacent RNA nucleotides
5. This forms phosphodiester bonds via condensation reactions
6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

Question 16

Describe how production of messenger RNA (mRNA) in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell

Answer 16

● Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
● Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells

Question 17

Describe how translation leads to the production of a polypeptide

Answer 17

1. mRNA attaches to a ribosome and the ribosome moves to a start codon
2. tRNA brings a specific amino acid
3. tRNA anticodon binds to complementary mRNA
   codon
4. Ribosome moves along to next codon and another
   tRNA binds so 2 amino acids can be joined by a
   condensation reaction forming a peptide bond
   ○ Using energy from hydrolysis of ATP
5. tRNA released after amino acid joined polypeptide
6. Ribosome moves along mRNA to form the
   polypeptide, until a stop codon is reached

Question 18

Describe the role of ATP in translation

Answer 18

● Hydrolysis of ATP to ADP + Pi releases energy
● So amino acids join to tRNAs and peptide bonds form between amino acids

Question 19

describe the role of ribosomes in translation

Answer 19

● mRNA binds to ribosome, with space for 2 codons
● Allows tRNA with anticodons to bind
● Catalyses formation of peptide bond between amino acids (held by tRNA molecules)
● Moves along (mRNA to the next codon) / translocation

Question 20

contrast the structures of DNA and mRNA molecules

Answer 20

1. DNA is double stranded/double helix and mRNA is single stranded
2. DNA is long, RNA is short
   3.thymine in DNA and uracil in RNA
   4.deoxyribose in DNA and ribose in RNA
   5.DNA has base pairing and mRNA doesn’t
   6.DNA has hydrogen bonding, mRNA doesn’t
   7.DNA has introns/non-coding sequences and mRNA doesn’t

Question 21

describe the role of tRNA in the process of translation

Answer 21

1.tRNA anticodon complementary to codon
2.tRNA brings specific amino acid
3.amino acids are carried/transferred to ribosome
4.allows for correct sequence of amino acids along polypeptide
5.tRNA works with ribosome to translate mRNA
6.tRNA transfer free amino acids from the cytoplasm to the ribosome to begin protein synthesis

Question 22

Describe how the base sequence of nucleic acids can be related to the
amino acid sequence of polypeptides when provided with suitable data

Answer 22

● You may be provided with a genetic code to identify which
triplets / codons produce which amino acids (example shown)
● tRNA anticodons are complementary to mRNA codons
○ Eg. mRNA codon = ACG → tRNA anticodon = UGC
● Sequence of codons on mRNA are complementary to sequence
of triplets on DNA template strand
○ Eg. mRNA base sequence = ACG UAG AAC
→ DNA base sequence = TGC ATC TTG
● In RNA, uracil replaces thymine

Question 23

what is the mistake here
“In transcription, polymerase joins
nucleotides.”

Answer 23

Be specific - RNA polymerase joins RNA nucleotides. Don’t confuse
this with DNA replication - DNA nucleotides and DNA polymerase.

Question 24

true or false “RNA polymerase forms hydrogen bonds / joins together complementary bases.”

Answer 24

false RNA polymerase joins adjacent (next to, not opposite) RNA nucleotides, forming phosphodiester bonds.

Question 25

What is a gene mutation?

Answer 25

● A change in the base sequence of DNA (on chromosomes) ● Can arise spontaneously during DNA replication (interphase)

Question 26

What is a mutagenic agent?

Answer 26

A factor that increases rate of gene mutation, eg. ultraviolet (UV) light or alpha particles

Question 27

Explain how a mutation can lead to the production of a non-functional protein or enzyme

Answer 27

1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA 2. So changes sequence of amino acids in the polypeptide 3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids) 4. So changes protein tertiary structure (shape) of protein 5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Question 28

Explain the possible effects of a substitution mutation

Answer 28

1. DNA base / nucleotide (pair) replaced by a different base / nucleotide (pair) 2. This changes one triplet so changes one mRNA codon 3. So one amino acid in polypeptide changes ○ Tertiary structure may change if position of hydrogen / ionic / disulphide bonds change OR amino acid doesn’t change ○ Due to degenerate nature of genetic code (triplet could code for same amino acid) OR if mutation is in an intron so removed during splicing

Question 29

Explain the possible effects of a deletion mutation

Answer 29

1. One nucleotide / base (pair) removed from DNA sequence 2. Changes sequence of DNA triplets from point of mutation (frameshift) 3. Changes sequence of mRNA codons after point of mutation 4. Changes sequence of amino acids in primary structure of polypeptide 5. Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein 6. Changes tertiary structure / shape of protein

Question 30

Describe features of homologous chromosomes

Answer 30

Same length, same genes at same loci, but may have different alleles

Question 31

Describe the difference between diploid and haploid cells

Answer 31

● Diploid - has 2 complete sets of chromosomes, represented as 2n ● Haploid - has a single set of unpaired chromosomes, represented as n

Question 32

Describe how a cell divides by meiosis

Answer 32

1.homologous chromosome pair up 2.crossing over/chiasmata form 3.produces new combination of alleles 4.chromosomes separate 5.at random (due to independent assortment) 6.produces varying combinations of chromosomes 7.chromatids separated at meiosis 2 later

Question 33

Draw a diagram to show the chromosome content of cells during meiosis

Answer 33

In the example shown: ● Parent cell has 4 chromosomes = 2 homologous pairs ● These appear as X shapes due to DNA replication

Question 34

Explain why the number of chromosomes is halved during meiosis

Answer 34

Homologous chromosomes are separated during meiosis I (first division)

Question 35

Explain how crossing over creates genetic variation

Answer 35

● Homologous pairs of chromosomes associate / form a bivalent ● Chiasmata form (point of contact between (non-sister) chromatids) ● Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes ● Creating new combinations of (maternal & paternal) alleles on chromosomes

Question 36

Explain how independent segregation creates genetic variation

Answer 36

● Homologous pairs randomly align at equator → so random which chromosome from each pair goes into each daughter cell ● Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells

Question 37

Other than mutation and meiosis, explain how genetic variation within a species is increased

Answer 37

● Random fertilisation / fusion of gametes ● Creating new allele combinations / new maternal and paternal chromosome combinations

Question 38

Explain the different outcomes of mitosis and meiosis

Answer 38

1. Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells ○ As 1 division in mitosis, whereas 2 divisions in meiosis 2. Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid) whereas meiosis halves the chromosome number (eg. diploid → haploid) ○ As homologous chromosomes separate in meiosis but not mitosis 3. Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically varied daughter cells ○ As crossing over and independent segregation happen in meiosis but not mitosis

Question 39

Explain the importance of meiosis

Answer 39

● Two divisions creates haploid gametes (halves number of chromosomes) ● So diploid number is restored at fertilisation → chromosome number maintained between generations ● Independent segregation and crossing over creates genetic variation

Question 40

How can you recognise where meiosis and mitosis occur in a life cycle?

Answer 40

● Mitosis occurs between stages where chromosome number is maintained ○ Eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n) ● Meiosis occurs between stages where chromosome number halves ○ Eg. diploid (2n) → haploid (n)

Question 41

Describe how mutations in the number of chromosomes arise

Answer 41

● Spontaneously by chromosome non-disjunction during meiosis ● Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis ● So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)

Question 42

Suggest how the number of possible combinations of chromosomes in daughter cells following meiosis can be calculated

Answer 42

2^n where n = number of pairs of homologous chromosomes (half the diploid number)

Question 43

Suggest how the number of possible combinations of chromosomes following random fertilisation of two gametes can be calculated

Answer 43

(2^n)^2 where n = number of pairs of homologous chromosomes (half the diploid number)

Question 44

True or false “all mutations results in a change in amino acid sequence”

Answer 44

False Due to the degenerate nature of the genetic code, many mutations do not affect the amino acid sequence of a polypeptide

Question 45

True or false “All mutations are harmful”

Answer 45

A mutation may result in the formation of a new, advantageous allele that results in change in polypeptide that positively affects the properties of the protein, resulting in a selective advantage

Question 46

True or false “Crossing over create new alleles”

Answer 46

Crossing over creates new combinations of maternal and paternal alleles on chromosomes

Question 47

what does non-disjunction mean?

Answer 47

failed separation of homologous pairs or sister chromatids leading to incorrect number of chromosomes in gametes

Question 48

which point in the cell cycle are mutations most likely to occur?

Answer 48

-interphase -because DNA replication/synthesis occurs/longest stage

Question 49

suggest how a mutation can lead to the production of a protein that has one amino acid missing

Answer 49

-loss of 3 bases/triplet/codon -stop codon/codon formed -triplet for last amino acid is changed to stop codon