4.1 DNA, genes and chromosomes

Question 1

What is a gene?

Answer 1

A section of DNA that contains a specific sequence of bases that code for a polypeptide.

Question 2

What is a locus?

Answer 2

Location of a gene on a chromosome.

gene that occupies a fixed postion

Question 3

What is an allele?

Answer 3

An alternative form of a gene.

Question 4

How many chromosomes do humans have?

Answer 4

46 (23 pairs).

Question 5

What is a homologous chromosome?

Answer 5

(Two chromosomes that) carry the same genes;
.

Question 6

Homologous chromosomes are the same __________ and have exactly the same…..

Answer 6

Size, genes.

Question 7

What can be different about homologous chromosomes?

Answer 7

They may have different alleles.

Question 8

How is DNA stored?

Answer 8

As chromosomes in the nucleus.

Question 9

What is the shape of chromosomes in eukaryotic cells?

Answer 9

Linear.

Question 10

How is DNA arranged in chromosomes?

Answer 10

Coiled & wrapped around histones (proteins).

Question 11

Describe the DNA in prokaryotic cells.

Answer 11

• Stored in CIRCULAR chromosomes
• DNA is shorter
• DNA is NOT associated with proteins.

Question 12

Do mitochondria and chloroplasts have their own DNA?

Answer 12

YES.

Question 13

What is the DNA in mitochondria and chloroplasts used for?

Answer 13

Codes for enzymes that are essential for aerobic respiration + photosynthesis, respectively.

Question 14

How does mitochondria and chloroplast DNA compare to the DNA found in the nucleus of cells?

Answer 14

Mitochondria and chloroplast DNA is shorter and circular, and is NOT histone bound.

Question 15

What are 3 features of the DNA code?

Answer 15

• Degenerate
• Universal
• Non-overlapping.

Question 16

What is the purpose of start and stop codons?

Answer 16

• The start codon marks the site at which translation into protein sequence begins, and the stop codon marks the site at which translation ends.
• Ensures that cells produce the correct sequences of amino acids.

Question 17

Degenerate:

Answer 17

Multiple codons code for the same amino acid.

Question 18

Universal:

Answer 18

The same triplet of bases code for the same amino acid in all organisms.

Question 19

Non-overlapping:

Answer 19

• Each base in a gene is only part of ONE triplet of bases that codes for ONE amino acid.
• Therefore, each codon, or triplet of bases, is read as a discrete unit.

Question 20

What is an advantage of the genetic code being non-overlapping?

Answer 20

If a point mutation occurs, it will only affect 1 codon, and therefore only 1 amino acid.

Question 21

Introns:

Answer 21

• Non-coding regions of DNA.

Question 22

Are introns found in eukaryotic and prokaryotic DNA?

Answer 22

NO - ONLY eukaryotic.

Question 23

What happens to introns after transcription?

Answer 23

Removed (spliced) out of mRNA molecules.

Question 24

Exons:

Answer 24

The sections of DNA that DO code for amino acids.

Question 25

Genome:

Answer 25

An organism's complete set of DNA in one cell.

Question 26

Proteome:

Answer 26

The full range of proteins in one cell.

Question 27

Does the genome ever change?

Answer 27

NO.

Question 28

Does the proteome change?

Answer 28

Yes -> it is constantly changing, depending on which proteins are currently needed.

Question 29

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.

Answer 29

- Condensation reaction - Between phosphate and deoxyribose - Catalysed by DNA polymerase.

Question 30

Describe how the separation of strands occurs in semi-conservative DNA replication.

Answer 30

- DNA helicase - Breaks hydrogen bonds between complementary base pairs.

Question 31

Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.

Answer 31

- Chloroplast DNA is shorter - Chloroplast DNA has fewer genes - Chloroplast DNA is circular (nucleus DNA = linear) - Chloroplast DNA not associated with proteins (nuclear DNA is).

Question 32

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why.

Answer 32

- Triplets code for the same amino acid (degenerate) - Occurs in introns / non-coding sequence.

Question 33

Explain how the organic bases help to stabilise the structure of DNA.

Answer 33

- Hydrogen bonds between the base pairs holds two strands together - Many hydrogen bonds provides strength.

Question 34

Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule.

Answer 34

- tRNA is 'clover leaf shape', mRNA is linear; - tRNA has hydrogen bonds, mRNA does not; - tRNA has anticodon, mRNA has codon.

Question 35

Describe and explain a difference in the structure of mRNA and pre-mRNA.

Answer 35

1. mRNA has fewer nucleotides 1. mRNA has no introns (only exons) 2. Because of splicing.

Question 36

Why is the genetic code described as being universal?

Answer 36

In all organisms, the same triplet codes for the same amino acid.

Question 37

What is the process that removes base sequences from pre-mRNA to form mRNA?

Answer 37

Splicing.

Question 38

What is the evidence from the diagram that DNA is a polymer?

Answer 38

Repeating units / nucleotides / monomer / molecules.

Question 39

The genetic code is described as degenerate. What is meant by this? Use an example from Table 1 to illustrate your answer.

Answer 39

1. More than one codon codes for a single amino acid; 2. Suitable example selected from Table 1;