Genetic information, variation and relationships between organisms

Question 1

Eukaryotic DNA

Answer 1

Long, linear associated with proteins called histones

Tightly coiled into chromosomes (DNA molecule and its associated proteins)

Question 2

Prokaryotic DNA

Answer 2

Short, circular, not associated with proteins/histones

Question 3

DNA in mitochondria and chloroplasts

Answer 3

Similar to prokaryotic DNA - short, circular, not associated with proteins/histones

Question 4

Genes

Answer 4

Sequence of DNA bases that codes for the amino acid sequence of a polypeptide or a functional RNA molecule eg ribosomal RNAs and tRNAs

A gene occupies a fixed position, called a locus, on a particular DNA molecule

Question 5

Features of the genetic code

Answer 5

Sequence of DNA triplets (or mRNA codons) codes for sequence of amino acids

Universal; the same DNA base triplets code for amino acids in all living organisms

Non-overlapping so it is discrete - each base can only be used once and in only one triplet

Degenerate, so the same amino acid can be coded for by more than one base triplet

Question 6

DNA coding and non-coding in eukaryotes

Answer 6

Places between genes contain many non-coding sections of DNA (non-coding multiple repeats, same base sequence repeated multiple times) which do not code for any amino acids

Within genes, only exons code for amino acid sequences, which are separated by one or more non-coding sequences called introns

Question 7

Genome

Answer 7

The complete set of genes in a cell, including those in mitochondria and/or chloroplasts

Question 8

Proteome

Answer 8

The full range of proteins that a cell/genome is able to produce

Question 9

Alleles

Answer 9

Different version (sequences of bases/triplets) of the same gene

Question 10

Homologous pair of chromosomes

Answer 10

Same size chromosomes with the same genes, but different alleles

Question 11

Codon

Answer 11

Sequence of three mRNA bases that codes for a specific amino acid

Question 12

Anticodon

Answer 12

Sequence of three tRNA bases that are complementary to a codon

Question 13

Triplet

Answer 13

Sequence of three DNA bases that codes for a specific amino acid

Question 14

Protein synthesis: 2 stages

Answer 14

Transcription; production of mRNA from DNA within the nucleus

Translation; production of polypeptides from the sequence of codons carried by mRNA in the cytoplasm on ribosomes

Question 15

Messenger RNA (mRNA)

Answer 15

Made by transcription in the nucleus and acts as a template for translation in the cytoplasm

It is a straight chain molecule. Sequence of bases on RNA determines sequence of amino acids in polypeptide chains

Sequence of bases on RNA determined by sequence of bases on DNA (triplets - codons)

It is chemically unstable so it breaks down after a few days

Question 16

Transfer RNA (tRNA)

Answer 16

Carries an amino acid (binding site)

Single polynucleotide strand that is folded (3 hairpin loops) which is held together by hydrogen bonds

Has an anticodon (3 bases) which bases are complementary to mRNA codon

Each tRNA specific to one amino acid, in relation to its anticodon

Question 17

Similarities/differences between the structure of mRNA and tRNA molecules

Answer 17

Similarities: both single polynucleotide strand

Differences: mRNA single helix/straight whereas tRNA is folded into a clover shape.

mRNA is longer, variable length whereas tRNA is shorter

mRNA contains no paired bases or hydrogen bonds, whereas tRNA has some paired bases and hydrogen bonds

Question 18

Transcription

Answer 18

Occurs in the nucleus.

DNA double helix is unzipped by helicase and the hydrogen bonds are broken.

RNA nucleotides align next to their complementary bases on the template strand forming temporary hydrogen bonds (thymine is replaced by uracil in RNA)

RNA polymerase joins adjacent nucleotides in a condensation reaction forming phosphodiester bonds.

When RNA polymerase reaches stop codon, mRNA (prokaryotes) or pre-mRNA (eukaryotes) detaches from DNA

mRNA leaves nucleus via nuclear pore

Question 19

Post transcriptional modification

Answer 19

Eukaryotic genes contain exons (coding regions) and introns (non-coding regions)

Whole gene is transcribed to pre-mRNA, which contains introns and exons.

Splicing is where introns are removed and exons are spliced together in different combos for different proteins

Prokaryotic DNA doesn’t contain introns and mRNA is directly produced from DNA, no splicing involved

Question 20

Translation

Answer 20

Sequence of mRNA codons determines sequence of amino acids

tRNAs carry specific amino acids, in relation to their anticodon

At the ribosome tRNA anticodon binds to mRNA codon and hydrogen bonds are formed. The first codon is the start codon.

Two amino acids are joined by condensation, forming a peptide bond using energy from ATP.

tRNA detaches without its amino acid, ribosome moves along mRNA to next codon, which continues until stop codon (polypeptide is released)

Question 21

Role of ATP in translation

Answer 21

Hydrolysis of ATP to ADP + Pi releases energy

For the bond between the amino acid and its corresponding tRNA molecule - the amino acid attaches at amino acid binding site.

For peptide bond formation between amino acids

Question 22

Role of tRNA in translation

Answer 22

tRNA attaches to and transports a specific amino acid, in relation to its anticodon

tRNA anticodon has complementary base pairs to mRNA codon, forms hydrogen bonds

Two tRNAs bring amino acids together for the formation of a peptide bond

About 60 types of tRNAs to carry 20 different amino acids. Genetic code is degenerate.

Question 23

Role of ribosomes in translation

Answer 23

Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing

Allows peptide bonds to form between amino acids

Question 24

Gene mutation

Answer 24

A change in the base sequence of DNA (on chromosomes)

Can arise spontaneously during DNA replication (interphase)

Involves base deletion/substitution

Question 25

Mutation effects

Answer 25

Leads to the production of a non-functional protein/enzyme

Change in base/triplet sequence of DNA/gene

Changes sequence of codons on mRNA

Changes sequence of amino acids in the primary structure of the polypeptide

Changes position of hydrogen/ionic/disulfide bonds in tertiary structure of protein

Changes tertiary structure/shape of the protein (and active site if enzyme)

If enzyme, substrate can’t bind to active site and form an enzyme-substrate complex

Question 26

Base deletion

Answer 26

One nucleotide/base removed from DNA sequence

Changes triplet/codon sequence from the point of mutation (frameshift)

Changes sequence of codons on mRNA after point of mutation

Changes sequence of amino acids in primary structure of polypeptide

Changes position of hydrogen/ionic/disulphide bonds in tertiary structure of protein

Changes tertiary structure/shape of protein i.e. non-functional or new and superior

Question 27

Base substitution

Answer 27

Nucleotide/base in DNA replaced with another nucleotide/base

Change in one base; changes one triplet

1.) Changes one mRNA codon and one amino acid; sequence of amino acids in primary structure of polypeptide changes etc.

OR

2.) Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged

Question 28

Mutagenic agents

Answer 28

Increase the rate of gene mutation (above the rate of naturally occurring mutations) e.g. ultraviolet light or alpha particles

Question 29

Pre meiosis

Answer 29

Before meiosis starts, DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere.

Question 30

Meiosis I (first division)

Answer 30

Separates homologous pairs

Chromosomes arrange into homologous pairs

Crossing over (prophase I) creates genetic variation in gametes.

Independent segregation (metaphase I) increases genetic variation in gametes (2n)

Question 31

Meiosis II (second division)

Answer 31

Separates chromatids (n).

Creates 4 haploid cells (from a single diploid parent cell) that are genetically varied

Question 32

How meiosis creates genetic variation

Answer 32

Crossing over between homologous chromosomes. Alleles exchanged between chromosomes. Creates new combinations of maternal and paternal alleles on chromosomes

Independent segregation of homologous chromosomes. Random alignment of homologous pairs at equator; random which chromosome from each pair goes to each daughter cell. Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells

Random fertilisation when two gametes fuse to form a zygote

Question 33

Importance of meiosis

Answer 33

Two divisions – creates haploid gametes (half number of chromosomes)

Diploid number restored at fertilisation

Maintains chromosome number from one generation to the next

Independent segregation and crossing over creates genetic variation

Question 34

Mutations in the number of chromosomes – chromosome non-disjunction

Answer 34

Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II

One gamete has an extra copy of this chromosome and the other has none

Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)

Arises spontaneously

Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21

Question 35

Differences between the outcomes of mitosis and meiosis

Answer 35

Mitosis produces diploid cells whereas meiosis produces haploid cells; two divisions in meiosis whereby homologous chromosomes separate then chromatids separate, whereas one division in mitosis whereby only sister chromatids separate

Daughter cells genetically identical to each other and parent cell in mitosis whereas in meiosis, daughter cells are genetically varied. Crossing over and independent segregation during meiosis I whereas no crossing over in mitosis

Mitosis produces 2 daughter cells whereas meiosis produces 4 daughter cells. Two divisions in meiosis whereas only one division in mitosis