Chapter 1: DNA and Genetics

Question 1

Identify the function and structure of the Watson and Crick model of DNA

Answer 1

The Watson and Crick model of DNA successfully described the complex structure of a DNA molecule, a double helix, thanks to the previous work of Rosalind Franklin in 1953.

Question 2

Describe and draw a nucleotide

Answer 2

DNA has a similar structure in all organisms in that it is made up of smaller molecules called nucleotides. Nucleotide molecules have three parts: phosphate group
deoxyribose sugar
one of four nitrogen-rich bases (commonly called bases).
A= Adenine T= Thymine C= Cytosine and G= Guanine
The nucleotides are organised in a way that makes DNA a double helix. The shape of a double helix is like that of a twisted rope ladder. The uprights of the ladder are made of alternating phosphate and sugar groups.

Question 3

Use a diagram to show the base pairing in DNA strands

Answer 3

I can’t add a photo (You have to pay) so learn this one!

Question 4

Describe the relationship between chromosomes, DNA and genes

Answer 4

DNA makes up genes which make up chromosomes
Deoxyribonucleic acid (DNA) is found in every cell of living organisms. In eukaryotes, like us, it is found in the nucleus of the cell. It has two strands which twist to form a double helix.
DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group, and one of the four types of nitrogen bases. (A,T,C,G). A pairs with T, C pairs with G: complementary base pairing. DNA is packaged into tube shaped structures called chromosomes. A gene is a section of DNA that is responsible for a characteristic like eye colour or blood group. Humans have around 20 000 genes.

Question 5

Describe the steps of mitosis

Answer 5

Stage 1: Chromosomes replicate to become double-stranded (two chromatids).

Stage 2: Double-stranded chromosomes become visible (Shaped like an ‘X’).

Stage 3: Double-stranded chromosomes line up along the equator of the cell.

Stage 4: The chromosomes are pulled to opposite ends of the cell.

Stage 5: Two nuclei form, each with the same number of chromosomes as the parent cell.

Stage 6: Membranes form, separating the two nuclei into the two daughter cells.

Question 6

Identify the role of meiosis in producing sex cells

Answer 6

Meiosis produces gametes (eggs and sperm) that have half the number of chromosomes of the parent cell, which is 23 in humans. In fertilisation, when the egg and sperm unite, the chromosomes unite as well, creating the human genome of 46 chromosomes.

Question 7

Compare mitosis and meiosis

Answer 7

Mitosis produces two genetically identical “daughter” cells from a single “parent” cell and is a continuous process for growth and healing, whereas meiosis produces sex cells that are genetically unique from the parent which creates variation in offspring and contains only half as much DNA (23).

Question 8

Relate the human reproductive organs to their function in sexual reproduction. FEMALE ovary and uterus

Answer 8

Ovary: Releases eggs (ova) and produces the female sex hormone oestrogen and progestin.
Uterus: Fertilised egg implants itself in the lining of the uterus to continue growing. The baby develops here during pregnancy.

Question 9

Relate the human reproductive organs to their function in sexual reproduction. FEMALE oviduct and vagina

Answer 9

Oviduct: (Fallopian tube) Tubes connecting ovaries to the uterus. Fertilisation occurs here.
Vagina: The penis is inserted here during sexual intercourse. Sperm must swim to the oviduct to fertilise the egg/ovum.

Question 10

Relate the human reproductive organs to their function in sexual reproduction. MALE testes and epidymus

Answer 10

Testes: produce sperm (male sex cells/gametes)
Epididymis: Sperm mature here

Question 11

Relate the human reproductive organs to their function in sexual reproduction. MALE sperm duct and penis

Answer 11

Sperm duct: Carries sperm to penis
Penis: Transfers sperm into female via the vagina.

Question 12

Distinguish between dominant and recessive traits.

Answer 12

Dominant traits are always physically expressed when an allele is dominant, even if only one copy of the dominant trait exists in the genotype. Recessive traits are physically expressed only if both the connected alleles are recessive.

Question 13

Recognise that both alleles are identical in … individuals but … individuals have different alleles.

Answer 13

1. Homozygous E.g RR or rr
2. Heterozygous E.g Rr

Question 14

Use Punnett squares to determine the possible combinations of genes in offspring

Answer 14

Can’t add picture so learn this one!

Question 15

Describe the results of a Punnett square using the terms genotype and phenotype

Answer 15

A punnett square gives us the possible genotype results of a cross between two individuals, which is the genetic information carried by an individual. E.g Tt, TT or tt (Big T is the dominant trait while little t is recessive). The phenotype is the physical trait that will be observable in the organism, depending on the combination of the alleles and which is dominant. E.g red or white flower, straight or curly tail.

Question 16

Describe some effects of mutations in genes or chromosome number in humans (Silent and Missense)

Answer 16

Silent mutations induce a change in a single base and usually do not affect the individual.
Missense mutations produce a different protein that may produce a disease such as sickle cell anaemia.

Question 17

Describe some effects of mutations in genes or chromosome number in humans (Nonsense and Frameshift)

Answer 17

Nonsense mutations are when a gene sequence is incomplete hence the protein is as well. E.g cystic fibrosis
Frameshift mutations is where there is an insertion or deletion of a single base which causes all the information after to b e jumbled. E.g Crohn’s disease, cystic fibrosis, and certain types of cancer.

Question 18

Define genetic modification

Answer 18

The insertion of new genes into organisms to change their genetic information and characteristics.