Topic 4.1: DNA, genes and chromosomes

Question 1

What is a chromosome?

Answer 1

a thread like structure of DNA and histone proteins

Question 2

What is a gene?

Answer 2

A section of DNA on a chromosome that codes for a whole polypeptide

Question 3

What is the structure of a chromosome?

Answer 3

• threads composed of DNA
• DNA molecules combine with histone proteins.
• This coils to form the chromosomes

Question 4

What are homologous pairs of chromosomes?

Answer 4

Pairs of chromosomes that carry the same genes but not the same alleles

Question 5

DNA codes for…..

Answer 5

the primary sequence of amino acids in a polypeptide

Question 6

What does the term codon refer to?

Answer 6

the sequence of three bases on mRNA that codes for a single amino acid

Question 7

Why is the genetic code degenerate?

Answer 7

most amino acids can be coded for by more than one codon

Question 8

Why is the genetic code non-overlapping?

Answer 8

Each base in the sequence is only part of time one codon

Question 9

Why is the genetic code universal?

Answer 9

The same codon codes for the same amino acid in all organisms

Question 10

Describe transcription (briefly)

Answer 10

DNA is converted into mRNA prior to protein synthesis as mRNA is small enough to leave the nucleus and enter cytoplasm where the code is used to determine the sequence of amino acids in a polypeptide

Question 11

Contrast the structure of DNA, mRNA and tRNA

Answer 11

• DNA double polynucleotide chain, mRNA/tRNA single stranded chain
• DNA longest molecule, then mRNA, then tRNA
• DNA double helix, mRNA straight molecule, tRNA clover shaped

Question 12

What is the genome?

Answer 12

The complete set of genes in a cell including those in the mitochondria and chloroplasts

Question 13

What is the genome?

Answer 13

The complete set of genes in a cell including those in the mitochondria and chloroplasts

Question 14

Contrast transcription in eukaryotes and prokaryotes

Answer 14

• Eukaryotes: takes place in nucleus, prokaryotes: takes place in cytoplasm
•No splicing in prokaryotes as there are no introns
• Transcription and translation can take place at same time in prokaryotes

Question 15

What are introns?

Answer 15

Non coding sequences within genes

Question 16

What are the advantages of mRNA carrying the genetic code?

Answer 16

•it is smaller so can exit nucleopotes
• if DNA left nucleus it could be damaged by enzymes
• can make multiple copies of mRNA so protein synthesis is efficient

Question 17

Adaptations of mRNA

Answer 17

• Single stranded so can exit nucleus
• Has codons for precise transcription
• No introns

Question 18

Describe 3 differences between pre-mRNA and mRNA

Answer 18

• pre-mRNA in nucleus, mRNA in cytoplasm
• pre-mRNA is intermediate stage, mRNA used in protein synthesis
• pre-mRNA has introns, mRNA has been processed

Question 19

What are ribosomes made from?

Answer 19

rRNA and proteins

Question 20

What are the three main phases of translation?

Answer 20

• Initiation
• Elongation
• Termination

Question 21

What happens in the initiation stage of translation?

Answer 21

The mRNA attaches to the ribosome: ribosome finds the start codon AUG

The tRNA anticodon binds to the complementary mRNA codon, carrying amino acid methionine

Question 22

What happens in the elongation phase of translation?

Answer 22

tRNA brings the specific amino acid to the ribosome and a peptide bond forms between the two amino acids with the use of ATP

tRNA is released from the ribosome, the ribosome moves along the mRNA repeating the process

Question 23

What is the termination phase of translation?

Answer 23

When a stop codon is reached, the ribosome, mRNA and tRNA all separate and the polypeptide chain is terminated, folding into its 3d shape

Question 24

What are chromatids?

Answer 24

Clones sections of chromosomes. When split, they form two distinct chromosomes

Question 25

What are daughter chromatids?

Answer 25

Pairs of chromosomes that have split. They end up in separate cells in mitosis or meiosis II

Question 26

Diploid cells have…

Answer 26

homologous pairs of chromosomes (2 of each type of chromosome =2n)

Question 27

Meiosis results in the production of…

Answer 27

4 gametes (haploid cells). Each gamete has half the number of chromosomes compared to body cells

Question 28

How does independent segregation lead to genetic variation in meiosis?

Answer 28

The maternal and paternal chromosomes in the homologous pairs separate completely randomly into each daughter cell in meiosis 1 This provides a huge variety of different combinations of chromosomes in the gametes

Question 29

How does crossing over in meiosis 1 lead to genetic variation?

Answer 29

Parts of chromatids in the homologous pairs twist together with sections crossing over The chromatids still contain the same genes but have different combinations of alleles (Chromatids with swapped sections are know and recombinant)

Question 30

Random fertilisation further increases genetic variation by…

Answer 30

providing a huge variety of new combinations of chromosomes

Question 31

Describe the differences between mitosis and meiosis

Answer 31

• Mitosis produces 2 daughter cells, meiosis produces 4 • 1 division in mitosis, 2 in meiosis • Crossing over only in meiosis • Daughter cells in mitosis have same number of chromosomes as parent cells, daughter cells have half the number • Daughter cells in mitosis are clones of each other and parent cells, not in meiosis

Question 32

What is a mutation?

Answer 32

Any change to the quantity or base sequence of the DNA of an organism

Question 33

What is a gene mutation?

Answer 33

Any change to the nucleotide bases or the sequence of bases

Question 34

What is a chromosome mutation?

Answer 34

Changes to the quantity of DNA

Question 35

What is a substitution mutation?

Answer 35

when a nucleotide is replaced by another nucleotide that has a different base

Question 36

What are the consequences of substitution mutation?

Answer 36

• No change > code is degenerate so same amino acid is coded for, so same protein produced • A different amino acid is coded for > may cause reduced or no function • May result in coding for a stop sequence > leads to a truncated protein, which may or may not have some functions

Question 37

What is a deletion mutation?

Answer 37

When a nucleotide is lost from the DNA sequence, causing a frame shift. This can change all of the amino acids in the polypeptide after the mutation

Question 38

What are 3 mutagenic agents, and why?

Answer 38

• Viruses • Radiation • Chemicals They increase rate of mutations by causing damage to DNA

Question 39

What is non- disjunction?

Answer 39

When chromosomes fail to separate

Question 40

What can chromosome mutations lead to?

Answer 40

Complex syndromes with varying characteristics; different mutations lead to different conditions depending on the chromosome

Question 41

What is polyploidy?

Answer 41

When organisms have 3 or more complete sets of chromosomes (occurs mostly in plants)

Question 42

Describe 4 differences between gene and chromosome mutations

Answer 42

• Gene mutation affects 1 chromosome only, chromosome mutation affects many genes • Gene mutation changes nucleotide sequence, chromosome mutations changes number or structure of chromosomes • GM only affects 1 copy of gene, CM can change the number of copies of many genes