unit 3 part 1

Question 1

Define allele

List two examples of genes with multiple alleles.

Answer 1

Version of a gene coding for different variations of a specific trait.

Gene for eye colour has alleles that code for different shades to

Gene for blood group

Question 2

State the source of new alleles of a gene

Answer 2

New alleles are formed through mutation in DNA sequence
Mostly arise due to errors in DNA replication

Question 3

What are beneficial, detrimental or neutral mutations.

Answer 3

Beneficial - new variations of trait
Detrimental - abrogate normal functioni of trait
Neutral (silent) - no effect

Question 4

Describe base substitution mutation

Answer 4

A single base was replaced with another in the gene sequence

Question 5

State the cause of sickle cell anemia, including the name of differences in Hb alleles

State the difference in amino acid sequences in sickle cell disease

Answer 5

Cause - change in the 6th codon in beta chain for haemoglobin
DNA sequence changes from GAG to GTG on non transcribed strand, CTC to CAC in transcribed strand
mRNA sequence changes from GAG to GUG

The sixth amino acid changed from glutamic acid (Glu) to valine (Val)

Question 6

Outline the consequences of the sickle cell mutation

Answer 6

Amino acid change alters the structure of haemoglobin to form insoluble fibrous strands
Haemoglobin cannot carry oxygen as effectively - tiredness
Changes to sickle shape
Sickle cells may form clots within capillaries, blocking blood supply to organs
Sickle cells are also destroyed more rapidly = low red blood cell count

Question 7

What is a degenerative codon

Answer 7

Codon that codes for the same amino acid as another codon

Question 8

State the aim of the human genome project
Outline two outcomes in the human genome project

Answer 8

To determine sequence of base pairs and location of genes

Established number, location, size and sequence of human genes
Production of specific gene probes to detect sufferers and carriers of genetic diseases
New and improved treatments due to discovery of new proteins
Comparisons with other genomes have provided insight into origins

Question 9

Compare the genetic material of prokaryotes and eukaryotes. ​

Answer 9

Prokaryote
Found freely in the cytoplasm
Naked
Genomes are compact
Extrachromosomal plasmids
Circular

Eukaryote
Contained in the nucleus
Bound to histone proteins
Genomes contain large amounts of non-coding and repetitive DNA
No plasmids
linear

Question 10

Describe the structure and function of plasmid DNA.​

Answer 10

Plasmids are small, circular DNA molecules that only contain a few genes and are capable of self-replication.
They are often found in prokaryotic cells.
Plasmids can self-replicate and autonomously synthesise proteins; they are ideal vectors for gene manipulation in labs.

Question 11

Describe the structure of eukaryotic DNA and associated histone proteins during interphase (chromatin).

Explain why chromatin DNA in interphase is said to look like “beads on a string.”

Answer 11

Linear
Associated with histone proteins
During interphase, the DNA is not supercoiled (chromatin)

Nucleosomes (DNA wrapped around histone proteins) are formed at regular intervals along the DNA strand.

Question 12

State the number of nuclear chromosome types in a human cell.​

Answer 12

Diploid = 46 chromosomes
Haploid = 23 chromosomes

Question 13

State a similarity and a difference found between pairs of homologous chromosomes.

Answer 13

Similarity
Same gene loci
Same centromere position
Same banding patterns

Differences
2 different origins
Different alleles of genes

Question 14

Describe the process of creating a karyogram.

Answer 14

Harvesting cells (usually from a foetus or white blood cells of adults)
Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not.
The chromosomes are stained and photographed and arranged into a karyogram.

Question 15

List the characteristics by which chromosomes are arranged on the karyogram.​

Answer 15

Size: arranged into homologous pairs according to size (sex chromosomes are shown last)
Banding pattern
Position of centromere

Question 16

Describe the relationship between the genome size of a species and the species complexity in structure, physiology and behavior.​

Answer 16

Genome size is not a valid indicator of genetic complexity.
As a general rule;
Viruses and bacteria have small genomes
Prokaryotes have larger genomes than eukaryotes
Plant genomes can vary in size dramatically

Question 17

Distinguish between a karyogram and a karyotype.

Answer 17

Karyograms = images of real chromosomes
Karyotypes = complete set of chromosomes in an individual

Question 18

Describe the use of a karyogram to diagnose Down syndrome.

Answer 18

If an individual has three copies of chromosome 21 (trisomy), the individual has down syndrome.

Question 19

Compare divisions of meiosis I and meiosis II.​

Answer 19

Meiosis I = separates pairs of chromosomes to halve the number of chromosomes (diploid to haploid)
Meiosis II = Separates sister chromatids (created by the replication of DNA during interphase) = 4 haploid daughter cells

Question 20

Define bivalent and synapsis.

Answer 20

Synapsis = paring of homologous chromosomes. Occurs during prophase I

Bivalent = bivalent (or tetrad) is a pair of synapsed homologous chromosomes.

Question 21

Outline the process and result of crossing over.​

Answer 21

Homologous chromosomes are held together at points called chiasmata.
Crossing over of genetic material between nonsister chromatids can occur at the chiasmata.
As a result of the exchange of genetic material, new gene combinations are formed on chromatids.
Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis.
If crossing over occurs, all four haploid daughter cells will be genetically distinct.