Chapter 8

Question 1

What is a gene

Answer 1

sections of DNA that contains code for making polypeptide and functional
RNA. code is specific sequence of bases.

Question 2

What do polypeptides make

Answer 2

POLYPEPTIDES MAKE PROTEINS AND SO GENES DETERMINE THE PROTEINS OF AN ORGANISMS.

Question 3

What is a locus

Answer 3

THE LOCATION OF A PARTICULAR GENE ON A CHROMOSOME IS CALLED THE LOCUS.

Question 4

What is an allele

Answer 4

AN ALLELE IS ONE OF A NUMBER OF ALTERNATIVE FORMS OF A GENE. MOST GENES OCCUR IN TWO, OCCASIONALLY MORE, DIFFERENT FORMS.

Question 5

IN A EUKARYOTIC CELL NUCLEUS, DNA IS STORED AS what

Answer 5

Chromosomes

Question 6

How many pairs of chromosomes do humans have

Answer 6

23 pairs of chromosomes

46 in total

Question 7

What is a homologous pair

Answer 7

PAIRS OF MATCHING CHROMOSOMES
ARE CALLED HOMOLOGOUS PAIRS (FOR
EXAMPLE THE TWO COPIES OF CHROMOSOME I ARE
HOMOLOGOUS PAIRS).

[Pairs of matching chromosomes are called homologous pairs. A homologous pair of chromosome are exactly the same size, have exactly the same genes BUT might have different alleles.]

Question 8

What is DNA stored as?

Answer 8

DNA is stored as chromosomes inside of the nucleus.

Question 9

What shape are chromosomes in eukaryotic cells

Answer 9

Chromosomes in eukaryotic cells are linear in shape.

Question 10

What is a nucleosome

Answer 10

This complex on DNA wrapped around a histone is called a nucleosome.

Question 11

How does the DNA fit into the nucleus

Answer 11

To tightly coil the DNA to fit in the nucleus as chromosomes, the DNA is tightly wound around proteins called histones.

Question 12

DNA in prokaryotic cell

Answer 12

Prokaryotes also carry DNA in chromosomes, BUT the DNA molecules are shorter and circular. The DNA is NOT wound around histones. Instead it supercoils to fit in the cell

[In prokaryotes DNA is coiled and stored in chromosomes, but the chromosomes are not protein bound. Chromosomes in prokaryotic cells are short and circular in shape.]

Question 13

DNA in chloroplasts and mitochondria

Answer 13

There is DNA found in chloroplasts and mitochondria. This DNA is similar to prokaryotic DNA, in that it is short and circular.

Question 14

3 features of genetic code

Answer 14

Degenerate

Universal

Non-overlapping

Question 15

What is a start condon

Answer 15

Start Codon
At the start of every gene there is a ‘start codon’ TAC in DNA or AUG in mRNA. This codes for the amino acid methionine. This methionine is later removed from the protein if it is not actually needed for the structure.

Question 16

What is a stop codon

Answer 16

Stop Codon
At the end of every gene there are 3 bases that do no code for an amino acid and is known as a
‘stop codon’. These stop codons mark the end of a polypeptide chain and cause ribosomes to detach and therefore stop translation. These codons are ATT,ATC and ACT on DNA.

Question 17

Genetic code is degenerate

Answer 17

There are 20 amino acids that the genetic code has to be able to code for. There are four DNA bases, (GCTA), and therefore three bases are needed to make enough combinations to code for at least 20 amino acids. This can be proven mathematically (4n).

If one base coded for one amino acid this would only allow for 4 amino acids to be coded for. This is insufficient to code for 20 amino acids.

If two bases coded for one amino acid this would allow for 16 amino acids to be coded for (4x4 combinations of code). This is insufficient to code for 20 amino acids.

If three bases coded for one amino acid this would allow for 64 amino acids to be coded for (4 x 4 x 4 combinations of code).

Question 18

Degenerate - amino acids coded for by more than 1 base triplet

Answer 18

64 combinations is more than is needed to code for 20 amino acids, and as a result each amino acid is actually coded for by more than one triplet of bases. This is what is meant by the genetic code be degenerate. e.g. tyrosine is coded for by ATA and ATG.

This genetic code wheel enables you to work out all combinations of bases that code for each of the 20 amino acids.

Question 19

Advantage of genetic code being degenerate

Answer 19

This is an advantage as if a point mutation occurs, even though the triplet of bases will be different, it may still code for the same amino acid and therefore have no effect

Question 20

Genetic code is universal

Answer 20

The same triplet of bases codes for the same amino acid in all organisms, this is why the genetic code is described as being universal.

Question 21

Advantage of genetic code being universal

Answer 21

This is an advantage as if it means genetic engineering is possible.
E.g. inserting the human gene for insulin into bacteria

Question 22

Genetic code is non-overlapping

Answer 22

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.

ACG GCT TCA ACT

Question 23

Advantage of genetic code being non-overlapping

Answer 23

This is an advantage as if a point mutation occurs, it will only affect one codon and therefore one amino acid.

Question 24

What are introns

Answer 24

Introns are sections of DNA that do not code for amino acids and therefore polypeptide chains. Introns are found in eukaryotic DNA, but not in prokaryotic DNA.

These get removed, spliced, out of mRNA molecules.

Question 25

What are exons

Answer 25

Exons are the sections of DNA that do code for amino acids.

Question 26

Genome and proteome

Answer 26

The genome is an organisms complete set of DNA in one cell, where as the proteome is the full range of proteins in one cell.

The genome should never change, whereas the proteome of the cell is constantly changing depending on which proteins are currently needed.

The genome of organisms widely varies, for example bacteria contain on average 600,000 DNA base pairs where as humans contain 3 billion DNA base pairs.

Question 27

Where are proteins created

Answer 27

On ribosomes

Question 28

2 main stages in the production of proteins from the DNA code

Answer 28

I. Transcription - where one gene on the DNA is copied into mRNA.

2. Translation - where the mRNA joins with a ribosome, and corresponding tRNA molecules brings the specific amino acid the codon codes for.

Question 29

Where is A complementary mRNA copy of one gene on the DNA created

Answer 29

In the nucleus

Question 30

mRNA is much shorter than DNA so it is able to what?

Answer 30

mRNA is much shorter than DNA so it is able to carry the genetic code to the ribosome in the cytoplasm to enable the protein to be made.

Question 31

Transcription process

Answer 31

I. The DNA helix unwinds to expose the bases to act as a template.

2. Only one chain of the DNA acts as a template.
3. Like with DNA replication, this unwinding and unzipping is catalysesd by DNA helicase.
4. DNA helicase breaks the hydrogen bonds between bases.
5. Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.
6. The enzyme RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied.

Once copied, the mRNA is modified and then leaves the nucleus through the nuclear envelope pores.

Question 32

What happens following transcription

Answer 32

Following transcription, pre-mRNA has to be modified to become mRNA that is ready to leave the nucleus and take part in translation.

The introns are spliced out by a protein called a splicesome. This leaves behind just the exons, the coding regions.

Question 33

What happens in translation

Answer 33

This is the stage in which the polypeptide chain is created using both the mRNA base sequence and the tRNA.

Question 34

Translation process

Answer 34

This is an advantage as if a point mutation occurs, it will only affect one codon and therefore one amino acid.