Genetic information, variation and relationships between organisms

Question 1

Describe the structure of DNA.

Answer 1

DNA has a double helix structure, it is formed of two separate strands which wind around one another to form a spiral.
They’re made up of lots of nucleotides joined together in a long chain.
DNA molecules are long and coiled up very tightly - therefore a lot of genetic information can fit into a small space.

Question 2

What is meant by complimentary base pairing?

Answer 2

Two DNA polynucleotide strands join together by hydrogen bonds. Each base can only join with its particular partner, this is called complimentary base pairing.

Question 3

What are some comparisons between DNA and RNA.

Answer 3

DNA is double stranded, RNA is single stranded.
DNA has a deoxyribose sugar, RNA has a ribose sugar.
DNA has bases A,T,G,C, RNA has bases A,U,G,C.
DNA is long, RNA is short.

Question 4

How is DNA stored in eukaryotic cells?

Answer 4

Eukaryotic cells contain linear DNA molecules that exist as chromosomes (found in the nucleus) which are thread like structures, each made up of one long molecule of DNA and its associated proteins.
DNA is wound around proteins called histones.
Histone proteins also help to support the DNA, the DNA and proteins are then coiled up very tightly to make a compact chromosome.

Question 5

How is DNA stored in prokaryotic cells?

Answer 5

Prokaryotic cells also carry DNA as chromosomes but the DNA molecules are shorter and circular.
The DNA isn’t wound around histones - it condenses to fit in the cell by supercoiling.

Question 6

What is the function of DNA?

Answer 6

Stores genetic information.

Question 7

What is the function of RNA?

Answer 7

Transfer genetic information from the DNA to the ribosomes.

Question 8

What are the monomers of DNA and RNA called?

Answer 8

Nucleotides.

Question 9

Which organelle is responsible for protein synthesis?

Answer 9

Ribosomes.

Question 10

What is a gene?

Answer 10

A short sequence of DNA that codes for a specific polypeptide.

Question 11

How are genes coded?

Answer 11

The order of the bases- each amino acid is coded for by a sequence of 3 bases called a triplet or codon.

Question 12

What is functional RNA?

Answer 12

Genes that don’t code for a polypeptide code for a functional RNA instead.
Functional RNA are RNA molecules other than mRNA, which perform special tasks during protein synthesis (e.g. tRNA, and ribosomal RNA which forms part of ribosomes).

Question 13

What is a genome?

Answer 13

The complete set of genes in a cell.

Question 14

What is a proteome?

Answer 14

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

Question 15

What is Non-coding DNA?

Answer 15

In eukaryotic cells a lot of nuclear DNA doesn’t code for polynucleotides.
Even genes that do code for polypeptides contain sections that don’t code for amino acids, these sections are called introns (their purpose isn’t known for sure).
Introns in eukaryotes are removed during protein synthesis - so they don’t affect the amino acid order.
Prokaryotic DNA don’t not have introns.
All the coding parts of a gene for amino acids are called exons.
Eukaryotic DNA also contains regions of multiple repeats outside of genes, these are DNA sequences that repeat over and over, for example CCTTCCTTCCTTCCTT. These areas don’t code for amino acids either, so they’re called non-coding multiple repeats.

Question 16

Alleles.

Answer 16

A gene can exist in more than one form.
The order of bases in each allele is slightly different, so code for slightly different versions of the same polypeptide.

Question 17

Homologous chromosomes.

Answer 17

In a eukaryotic cell nucleus, DNA is stored as chromosomes.
Humans have 23 pairs.
Pairs of matching chromosomes are called homologous pairs.
In a homologous pair both chromosomes are the same size and have the same genes - although they 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 18

What do three bases code for?

Answer 18

An amino acid.

Question 19

What is a locus on a chromosome?

Answer 19

A fixed position where a particular allele is found.

Question 20

What is protein synthesis and what are the two main stages?

Answer 20

Protein synthesis is the production of proteins (polypeptides) from the information contained with a cell’s DNA (also known as polypeptide synthesis).
The two main stages are:
Transcription-Where the DNA code is copied into a molecule called mRNA.
Translation-Where mRNA joins with a ribosome and the code it carries is used to synthesis a protein.

Question 21

Messenger RNA.

Answer 21

mRNA is made during transcription.
It carries the genetic code from the DNA to the ribosomes, where it is used to make a protein during translation.
mRNA is a single polynucleotide strand. In mRNA, groups of three adjacent bases are usually called codons.

Question 22

Transfer tRNA.

Answer 22

tRNA is involved in translation.
It carries amino acids to the ribosomes.
tRNA is a single polynucleotide strand that’s folded into a clover shape. Hydrogen bonds between the specific bases pairs hold the molecule in this shape.
Every tRNA molecule has a specific sequence of three bases at one end called an anticodon (sequence that is complimentary to a corresponding codon). It also has an amino acid binding site at the other end.

Question 23

What is an anticodon?

Answer 23

A specific sequence of 3 bases on tRNA that is complimentary to a corresponding codon on mRNA that codes for a specific amino acid.

Question 24

What is an mRNA codon?

Answer 24

Group of three adjacent bases.

Question 25

Transcription.

Answer 25

During transcription an mRNA copy of a gene is made from DNA. -In eukaryotic cells transcription takes place in the nucleus, in prokaryotic cells it takes place in the cytoplasm. -It happens in four steps: 1) RNA polymerase attaches to the DNA. 2) Complementary mRNA is formed. 3) RNA polymerase moves down the DNA strand. 4) RNA polymerase reaches stop signal.

Question 26

What happens in step 1 of transcription?

Answer 26

- Transcription starts when RNA polymerase attaches to the DNA double helix at the beginning of a gene. - In eukaryotes the hydrogen bond between two DNA strands in the gene are broken by DNA helicase attached to the RNA polymerase. - This separates the strands, and the DNA molecules uncoils at that point, exposing some of the bases. - One of the strands is then used as a template to make an mRNA copy.

Question 27

What happens in step 2 of transcription?

Answer 27

- 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. - Specific complementary base pairing means that the mRNA strand ends up being a complimentary copy of the DNA template strand (except the base T is replaced by U in RNA). - Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they're joined together by RNA polymerase forming an mRNA strand.

Question 28

What happens in step 3 of transcription?

Answer 28

- The RNA polymerase moves along the DNA, assembling the mRNA strand. - The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double helix.

Question 29

What happens in step 4 of transcription?

Answer 29

- When RNA polymerase reaches a particular sequence of DNA called a stop signal, it stops making mRNA and detaches from the DNA. - In eukaryotes, mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, where the next stage of protein synthesis takes place.

Question 30

Editing mRNA.

Answer 30

- Transcription produces different products in eukaryotes and prokaryotes. In eukaryotes the introns and exons are both copied into mRNA during transcription. - mRNA strands containing introns and exons are called pre-mRNA. In a process called splicing introns are removed and the exons join together, forming mRNA strands. This takes place in the nucleus. The mRNA then leaves the nucleus for the next stage of protein synthesis.

Question 31

Define transcription.

Answer 31

A DNA gene is copied into mRNA.

Question 32

Describe the structure of mRNA.

Answer 32

Single stranded polynucleotide.

Question 33

What is the function of tRNA?

Answer 33

Carries amino acids to make proteins to ribosomes during translation.

Question 34

Describe splicing.

Answer 34

Introns are removed from pre-mRNA and exons joined together.

Question 35

Translation.

Answer 35

Second stage of protein synthesis. Occurs at the ribosomes in both eukaryotes and prokaryotes. During translation amino acids are joined together to make a polypeptide chain (protein) following the sequence of codons carried by mRNA.

Question 36

What happens in step 1 of translation?

Answer 36

- mRNA attaches to ribosome, ribosome finds the start codon. - tRNA carry amino acids to it. - ATP provides energy needed for the bond between the amino acid and the tRNA molecule to form.

Question 37

What happens in step 2 of translation?

Answer 37

- A tRNA molecule, carrying an amino acid with an anticodon that's complimentary to the first codon on the mRNA attaches itself to the mRNA by complimentary base pairing. - Ribosome moves along to next codon. - A second tRNA molecule attaches to the next codon on the mRNA.

Question 38

What happens in step 3 of translation?

Answer 38

- The two amino acids attached to the tRNA are joined by a peptide bond (using ATP). - The first tRNA moves away, leaving the amino acid behind.

Question 39

What happens in step 4 of translation?

Answer 39

- A third tRNA molecule binds to the next codon on the mRNA. - It's amino acid bind to the first two and the second tRNA molecule moves away. - This continues producing a chain of amino acids (polypeptide chain) until there is a stop signal.

Question 40

What happens in step 5 of translation?

Answer 40

- The polypeptide chain (protein) then moves away and translation is complete.

Question 41

Describe what is meant by the genetic code is non-overlapping.

Answer 41

The genetic code is the sequence of base triplets (codons) in mRNA which code for specific amino acids. The base triplets are read in sequence. They do not share their bases - the code is non-overlapping.

Question 42

Describe what is meant by the genetic code is degenerate.

Answer 42

The genetic code is also degenerate - there are more possible combinations of triplets than there are amino acids. 20 amino acids but 64 possible triplets - meaning some amino acids are coded for by more than one base triplet. For example, tryosine can be coded for by UAU or UAC. Not all triplets code for amino acids: - Stop signals (at the end of mRNA) that tell the cell to stop production of a protein. - Start signals at the start of mRNA which tell the cell to start a new protein.

Question 43

describe what is meant by the genetic code is universal.

Answer 43

The genetic code is universal. The same base triplets code for the same amino acids in all living things.

Question 44

What is meant by non-coding multiple repeats?

Answer 44

Regions outside of coding sections, multiple repeats of bases.

Question 45

What is a locus?

Answer 45

A fixed point on a chromosome where an allele is found.