genetics, biodiversity and classification

Question 1

what are DNA molecules like in prokaryotic cells and in mitochondria and chloroplasts?

Answer 1

short, circular and not
associated with proteins.

Question 2

what are DNA molecules like in eukaryotic cells? (2)

Answer 2

1) found in the nucleus
2) long, linear and associated with proteins called histones to form chromosomes

Question 3

when are chromosomes first visible and what are they made of? (4)

Answer 3

1) at the start of cell division
2) is the result of the DNA being tightly coiled around the histones
3) each thread is called a chromatid
4) DNA is highly coiled and folded to form the chromosome

Question 4

what is a gene? (3)

Answer 4

1) a base sequence of DNA that codes for:
2) the amino acid sequence of a polypeptide
3) a functional RNA (including ribosomal RNA and tRNAs).

Question 5

where are genes found? (2)

Answer 5

1) genes occupy a fixed position, called a locus
2) on a particular DNA molecule

Question 6

what is a triplet?

Answer 6

A sequence of three DNA bases

Question 7

what does a triplet code for?

Answer 7

a specific amino acid

Question 8

how can you describe the genetic code? (4)

Answer 8

1) universal- each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution (with a few minor exceptions)
2) non-overlapping- each base in the sequence is only read once as triplets.
3) degenerate- most amino acids are coded for by more than one triplet
4) The genetic codes contains start and stop codons.

Question 9

what are exons? (2)

Answer 9

1) A sequence of DNA that codes for an amino acid sequence. within a gene there are only some exons.
2) Within the gene, these exons are separated by one or more non-coding sequences, called introns.

Question 10

In eukaryotes, much of the nuclear DNA does not code for (1)
There are, for example (2) of base sequences between (3)

Answer 10

1) polypeptides
2) non-coding multiple repeats
3) genes

Question 11

what is the genome?

Answer 11

the complete set of genes in a cell

Question 12

what is the proteome?

Answer 12

the full range of proteins that a cell is able to produce.

Question 13

what is a codon?

Answer 13

a sequence of three bases on mRNA that codes for a specific amino acid.

Question 14

what is an anti codon

Answer 14

​a sequence of three nucleotide bases at one end of a tRNA molecule that is specific to an mRNA codon.

Question 15

what is a homologous pair? (2)

Answer 15

1) two chromosomes that carry the same genes on the same location
2) they are not identical as they can carry different alleles of the same gene.

Question 16

why is each amino acid represented by more than one triplet? (3)

Answer 16

1) there are four nucleotide bases which code for 20 different amino acids
2) Scientists were able to conclude that each amnio acid is coded for by one or more combination of triplets. 3) There are therefore 64 possible triplets (4^3) meaning that each amino acid is represented by more than one triplet.

Question 17

what is an allele?

Answer 17

an alternative form of the same gene

Question 18

what is the structure of mRNA? (5)

Answer 18

1) a long single polynucleotide chain strand arranged in a single helix, created during transcription in which the base sequence is complementary to DNA.
2) Each set of three bases, called the codon, match a triplet on the DNA as well as the tRNA.
3) pentose sugar is ribose
4) molecule is smaller than DNA but larger than tRNA
5) organic bases are adenine, uracil, guanine and cytosine

Question 19

what is the structure of tRNA? (6)

Answer 19

1) a small molecule made up of around 80 nucleotides with bases U, A C and G
2) a single polynucleotide chain that is folded into a clover leaf shape in which one end extends from the others where an amino acids attaches
3) At the opposite end is an anti- codon which is specific to the amino acid that the tRNA attaches to
4) anticodon can recognize and decode an mRNA codon
5) pentose sugar is ribose
6) many types of tRNA each of which binds to a specific amino acid

Question 20

what is transcription in simple terms?

Answer 20

the production of mRNA from DNA

Question 21

outline the stages of transcription (4)

Answer 21

1) hydrogen bonds between the complementary bases break due to the action of an enzyme and the DNA uncoils, separating the two strands exposing the organic bases.
2) one of the DNA strands is used as a template to make the mRNA molecule, the template
is called the antisense strand.
3) free nucleotides line up by complementary base pairing and adjacent nucleotides are joined by phosphodiester bonds thus forming a molecule of mRNA. The enzyme RNA polymerase catalyses the formation of phosphodiester bonds joining mRNA nucleotides together. When a stop codon is reached this ceases. As the RNA polymerase moves away the DNA rejoins, with only 12 bases being exposed at a time to reduce the chance of damage to the DNA
4) In eukaryotic cells the pre-mRNA is then spliced to remove the introns leaving just a strand of exons. The mRNA then moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm which is the site of next stage of protein synthesis called translation.

Question 22

why do different species produce some unique proteins?

Answer 22

each genome is specific to different species

Question 23

what does transcription result in, in prokaryotes?

Answer 23

the production of mRNA from DNA.

Question 24

what does transcription result in, in eukaryotes?

Answer 24

the production of pre- mRNA; this is then spliced to form mRNA

Question 25

what is translation in simple terms?

Answer 25

the production of polypeptides from the sequence of codons carried by mRNA