Genetics

Question 1

Describe the structure and function of DNA

Answer 1

Structure:
-double stranded molecule made of 2 nucleotides
-nucleotides has 3 units = sugar, phosphate group and a base
-DNA’s sugar = deoxyribose
-one of the bases is joined to another on the sugar/phosphate chains which forms the steps of the ladder, and the two chains are twisted around which gives the double helix shape
-double helix- twisted more around histones (the structural proteins which are important in maintaining the heavily coiled 3-d shape

Function:
-store and transmit genetic information
-it serves as the information to direct and regulate the construction of the proteins necessary for the cell to preform all of its functions

Genetic code:
The four bases of DNA: adenine (A), guanine (G), cytosine (C), thymine (T)
-arranged in precise order so its easy to read when protein synthesis is required
-complementary base pairing- each base along one strand of DNA pairs with another on the other side (A and T, C and G)
-the bases opposite to each other are held together by hydrogen bonds

Question 2

Describe the structure and function of RNA

Answer 2

Structure:
-type of nucleic acid called ribonucleic acid
-the nucleotides rejoined together to form a single strand
-nucleotides are held together by strong covalent bonds between the sugar and of one nucleotide and the phosphate of the other
-single stranded
-has sugar called ribosome
-has the base uracil (U) instead of thymine

Function:
-controls the assembly of amino acids into proteins
-create proteins through translation

Question 3

Describe the structure and function of genes

Answer 3

Structure:
-genes normally exist as pairs because the gene on one chromosome is matched at the equivalent site (lotus) on the other chromosome of the pair

Function:
-code for proteins (structural proteins or enzymes)
-code for RNA
-regulating (turns genes ‘on’ or ‘off’)

Question 4

Describe the structure and function of chromosomes

Answer 4

Structure:
-23 pairs of chromosomes - 22 autosomes, 1 is longest and 22 the shortest, 1 pair sex chromosome
-made of a tightly coiled strand of DNA, with supporting proteins called histones
-if they were uncoiled it would be around 2 metres in each body cell
-each end of the chromosome is capped with a length of DNA called ‘telomere’- seals chromosomes and are structurally essential
-in replication, the telomere is shortened and this damages the DNA so an enzyme called telomerase fixes it

Function:
-carry genetic information
-helps ensure that DNA is replicated and distributed appropriately during cell division

Question 5

Define allele

Answer 5

Different forms of the same gene

Question 6

Define locus

Answer 6

The physical site or location of a specific gene on a chromosome

Question 7

Define genotype

Answer 7

The genetic makeup of an organism; the gene or allele combination an organism has

Question 8

Define phenotype

Answer 8

The physical characteristics of an organism, the way an organism looks

Question 9

Define homozygous

Answer 9

Two identical forms of the gene

Question 10

Define heterozygous

Answer 10

Two different forms of a gene

Question 11

Define dominance

Answer 11

A gene allele that ‘expresses’ over another allele in homozygous and heterozygous pairs, shown in the phenotype

Question 12

Define co-dominance

Answer 12

Condition in which both allele of a gene are expressed in hetrozygous offspring

Question 13

Define recessive

Answer 13

A gene allele that only ‘expresses’ when it’s matched with another recessive allele, most of the time only works with homozygous recessive

Question 14

Define homologous chromosomes

Answer 14

One pair of a pair of chromosomes with the same gene sequence, loci, chromosomal length and centromere location

Question 15

Define mutation

Answer 15

Is a change in an organism’s DNA

Question 16

Define autosome

Answer 16

Are the first 22 homologous pairs of chromosomes that do not determine the sex of the individual

Question 17

Define sex chromosomes

Answer 17

Are the 23rd pair of chromosomes that determine the sex of the individual

Question 18

Draw a punnet square indicating the genotypic and phenotypic traits to demonstrate the inheritance of autosomal dominant (e.g. polycystic kidney disease), autosomal recessive (e.g. PKU, sickle cell anaemia, and thalassaemia) and sex linked traits (e.g. haemophilia and colour blindness)

Answer 18

1. Autosomal dominant: is when the mutated gene is the dominate gene located on one of the non sex chromosomes e.g. Huntington disease (print out)
2. Autosomal recessive: is when the mutate gene is the recessive gene located on one of the non sex chromosomes. Two copies of the gene need to be present to be able to show in the offspring e.g. phenylketonuria (PKU)
3. Sex linked: sex- linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes e.g. haemophilia A

Question 19

Discuss co-dominance in relation to the ABO blood group

Answer 19

There are 4 blood types (phenotypes) resulting from 6 possible combinations of 3 different alleles of a single gene
Genotype and phenotype:
I^AI^A or I^Ai = A
I^BI^B or I^Bi = B
I^AI^B = AB
ii = O

See punnet chart print out to see how parents with certain blood groups have children with different blood groups

Question 20

Differentiate between chromosome, single gene, polygenic and multifactorial disorder

Answer 20

Chromosome: a package of DNA with part or all of the genetic material of an organism.
Polygenic: describes a characteristic that is influenced by many genes e.g. eye colour, hair colour. Trait controlled by more than one gene
Multifactorial: referring to a phenotypic character that is influenced by multiple genes and environmental factors e.g. cigarette smoke, sunlight
Single gene: trait controlled by one gene that had two alleles

Question 21

Differentiate between mitosis and meiosis

Answer 21

Mitosis:
-type of cell division in which diploid body cell copies itself and finally divides into identical diploid daughter cells
-daughter cells are clones of each other and in each base pair, their DNA is identical
1. Interphase: cell spends most of its life in this phase. The dna in chromosomes copies itself ready for mitosis
2. Prophase: the dna in chromosomes and their copies condenses to become more visible. The membrane around the nucleus disappears
3. Metaphase: chromosomes and their copies line up in the middle of the cell
4. Anaphase: chromosomes and their copies are pulled to different ends of the cell
5. Telophase: new membranes form around the chromosomes at each end of the cell
6. Cytokinesis: the cell membrane pinches in and eventually divides into two daughter cells

Meiosis:
-cell division that produces gametes
-human body cell = 46 chromosomes arranged in 23 pairs, gametes are haploid- only has a single set of 23 unpaired chromosomes
-produces four genetically different haploid cells, so its a reduction division which means the chromosome number is halved from diploid to haploid
-parent cell -> chromosomes make identical copies of themselves -> similar chromosomes pair up -> sections of DNA get swapped -> pairs of chromosomes divide -> then chromosomes divide

Meiosis vs mitosis:
-mitosis: one division forming 2 identical cells (clones); meiosis: two divisions forming 4 genetically different cells
-meiosis has 2 rounds of genetic separation and cellular division while mitosis only has one of each. In meiosis homologous chromosomes separate leading to daughter cells that are not genetically identical. In mitosis the daughter cells are identical to the parent as well as to each other
-mitosis is normal cell division everywhere. Meiosis happens only in sex cells (gametes)

Question 22

Look up a 3 generation family tree

Answer 22

Understand the pedigree symbols (print out)

Question 23

Describe the antenatal screening tests and their relevance to genetics: sickle cell trait, thalassemia, Down’s syndrome, neural tube defects etc

Answer 23

Extra reading

Question 24

Discuss some of the multifactorial genetic disorders

Answer 24

Extra reading

Question 25

Describe the role of the genetic counsellor and the specialist service

Answer 25

Extra reading