topic 4

Question 1

Describe the characteristics of nuclear eukaryotic DNA

Answer 1

-Linear
-Associated with histones
-Larger than prokaryotic DNA
-Really long so wound up and coiled tightly into chromosomes to fit into the nucleus

Question 2

Describe the characteristics of prokaryotic DNA

Answer 2

-shorter
-circular
-not associated with histones - condenses into cell by supercoiling

Question 3

What is a gene?

Answer 3

a sequence of DNA bases that codes for either a polypeptide or functional RNA

Question 4

What is a cell’s genome?

Answer 4

the complete set of genes in the cell

Question 5

What is a cell’s proteome?

Answer 5

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

Question 6

What are introns? What happens to them during protein synthesis?

Answer 6

Introns are sections of genes that do not code for amino acids
Introns are therefore not useful for protein synthesis and are spliced out of the gene

Question 7

What are exons?

Answer 7

sections of a gene that do code for amino acids

Question 8

What is an allele/

Answer 8

alleles are different forms of a gene. they code for slightly different versions of the same polypeptide
e.g. the gene for blood type exists in 3 different alleles - type A, B, and O

Question 9

What are homologous pairs of chromosomes?

Answer 9

pairs of chromosomes that are the same size, have the same genes, but could have different alleles. Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosome in a homologous pair

Question 10

What is mRNA? What does it do?

Answer 10

mRNA (messenger RNA) is made during transcription and it carries the genetic code from the DNA to the ribosomes, where its used to make a protein during translation

mRNA is a single polynucleotide strand, groups of 3 bases are called codons

Question 11

What is tRNA? What does it do?

Answer 11

tRNA (transfer RNA) is involved in translation - it carries the amino acids that are used to make proteins to the ribosomes.

tRNA is a single polynucleotide chain that’s folded into a clover shape. Hydrogen bonds between specific base pairs hold the molecule in this shape

Every tRNA molecule has a specific sequence of 3 bases at one end called the anticodon

They also have an amino acid binding site at the other end

Question 12

Describe the steps involved in transcription of protein synthesis

Answer 12

1. RNA polymerase attaches to the DNA double-helix at the beginning of a gene
2. Hydrogen bonds between the 2 DNA strands in the gene are broken by DNA helicase attached to the RNA polymerase. This separates the strands and the DNA molecule uncoils at that point, exposing some of the bases
3. One of the strands is then used as a template to make an mRNA copy
4. The RNA polymerase lines up free RNA nucleotides alongside the exposed bases on the template strand. The free bases are attracted to the exposed bases. Complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (expect the T is replaced with a U in RNA)
5. The nucleotides are joined with the DNA bases by RNA polymerase, forming phosphodiester bonds to form an mRNA molecule

6.RNA polymerase moves along DNA, separating the strands + assembling the mRNA strand

7. The hydrogen bonds between the uncoiled DNA strands reform once RNA polymerase has passed by and the strands coil back into a double-helix

8.When RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA

9.In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm.

Question 13

How is transcription different in eukaryotes and prokaryotes?

Answer 13

Eukaryotes - introns + exons both copied into mRNA (pre-mRNA). A process called splicing occurs where introns are removed from mRNA strand and the exons are joined together, making mRNA.

Prokaryotes - mRNA directly from DNA without splicing taking place, as there is no introns in prokaryotic DNA

Question 14

Describe the steps involved in translation of protein synthesis

Answer 14

1. The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it. ATP provides the energy needed for the bond between the amino acid and the tRNA molecule to form
2. A tRNA molecule, with an anticodon complementary to the first codon on the mRNA attaches itself to the mRNA by specific base pairing

3, A second tRNA molecule attaches itself to the next codon on the mRNA in the same way

4. The 2 amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecules moves away, leaving its amino acid behind
5. A 3rd tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first 2 and the second tRNA molecule moves away
6. This process continues, producing a polypeptide chain, until there’s a stop signal on the mRNA molecule
7. The polypeptide chain moves away from the ribosome and translation is complete

Question 15

What is the genetic code?

Answer 15

the sequence of base triplets (codons) in mRNA which code for specific amino acids

Question 16

Describe the characteristics of the genetic code

Answer 16

Non-overlapping = each base triplet is read in sequence, separate from the triplet before and after it.

Degenerate = there are more possible combinations of triplets than there are amino acids. Means some amino acids are coded for by more than 1 base triplet.

Universal = the same triplets code for the same amino acids in all living things