3.1.1 Definition of a gene

Question 1

What is a gene?

Answer 1

Inherited units of information that are able to affect the phenotype of an organism.

Question 2

What are the products of a gene?

Answer 2

mRNA that will then code for proteins or other forms of functional RNA like rRNA, tRNA, miRNA, etc

Question 3

• What were the pre-mendelian ideas of heredity?

Answer 3

17th century: Harvey, Swammerdam, De Graaf, Steno and Redi suggested that all females produce eggs and that ‘like breeds like’
In 1677 van Leeuwenhoek uses his magnifying lens to discover sperm
In 1694 Hartsoeker uses his own lenses to co-discover sperm and claims to see ‘miniature humans’ pre-formed in the sperm - HOMUNCULI
Charles Darwin in 1859 published his Origin of species, but was unaware of Mendel’s work so instead supported the idea of ‘blending’ the genes of the parents, but did eventually realise that this would result in a dead end.

Question 4

• What is the Mendelian idea of heredity?

Answer 4

Mendel, through systematic observations of characteristics (and good practical technique) in pea plants correctly predicted that characters could be controlled by specific factors, and that multiple characteristics could arise from one factor.
In this way the idea of genes and alleles arose.

Question 5

• How do alleles segregate?

Answer 5

Randomly

Question 6

• How do non-linked traits segregate?

Answer 6

Randomly and independently from each other

Question 7

• How do linked traits segregate?

Answer 7

They will be inherited together, although very rarely crossing over may occur and linked genes will show variation.

Question 8

• Experimental evidence for beginnings of molecular genetics

Answer 8

Friedrich Miescher, 1869 - found that the material inside a nucleus wasn’t entirely protein through experiments on pus-covered bandages (isolated the relatively large nuclei from neutrophil cells present), names it ‘nuclein’ but this is later renamed DNA.
Walter Flemming, 1882 - creates the term ‘chromatin’ to describe the material that becomes ‘stringy’ during cell division.
Theodore Boveri, 1890 - continues some of his earlier work on chromosomes to establish that sperm and egg cells contribute to an equal number of chromosomes.

Mendel’s work is rediscovered in the 1900s
Theodore Boveri and Walter Sutton, 1902 - suggest that chromosomes contain the ‘characters’ for characteristics described by Mendel.
Wilhelm Johannsen, 1909 - defines the basic unit of heredity, the gene
Avery, McCarty and McLeod, 1944 - show that DNA is the carrier of genes and therefore also inheritance.

Question 9

Where are the telomeres?

Answer 9

At the ends of the arms of the chromosome

Question 10

Where is the centromere?

Answer 10

At the point where the two sister chromatids join to form the characteristic X shape of a chromosome

Question 11

What are the different arms of the chromosome called?

Answer 11

Short arm (p) and long arm (q)

Question 12

How can the letters for the long and short arms of the chromosome be remembered?

Answer 12

p (short arm) for petite

q (long arm) is after p in the alphabet

Question 13

What does the shape of the chromosome depend on?

Answer 13

Where the centromere is located

Question 14

What makes up a mitotic chromosome?

Answer 14

Two identical copies of the same chromosome (there are two chromatids present)

Question 15

What is the shape of the chromosome known as when the centromere is high up?

Answer 15

Acrocentric (acro for ‘peak’ or ‘top of the hill’)

Examples include 13, 14, 15, 21, 22 and Y

Question 16

What is the shape of the chromosome known as when the centromere is a bit above the midpoint?

Answer 16

Submetacentric

Question 17

What is the shape of the chromosome known as when the centromere is in the middle?

Answer 17

Metacentric

Examples include 1, 3, 19 and 20

Question 18

Why is the location of the centromere important?

Answer 18

Because it can change if translocation mutations occur (will be covered in more detail elsewhere)

Question 19

What is a karyotype?

Answer 19

This is the sum of all the chromosomes present in a species - each species has a specific set, and similar looking organisms can have hugely varied karyotypes.

Question 20

How is staining of chromosomes characteristic?

Answer 20

It results in banding of the chromosome (light and dark regions) - the stain marks areas with higher concentrations of cytosine and guanine

Question 21

How is genetic information passed between generations?

Answer 21

Through the chromosomes, as these carry the genes - cell division is highly controlled and there are many checkpoints throughout the cell cycle that must be passed in order to prevent faults in the chromosomes being passed on to future generations.

Question 22

What is a cell known as if it enters G0 state?

Answer 22

Quiescent - it has left the cell cycle. This often occurs in fully differentiated cells, although some (like chondrocytes) are able to divide a few times whilst being full differentiated before entering a quiescent state.

Question 23

Where are the chromosomes kept?

Answer 23

Within the nucleus - they appear to exist as a ‘large bowl of spaghetti’/intertangled and random, as condensed chromosomes only exist during M phases/cell division 9 in order to limit breakages). Instead, each chromosome has been shown to occupy a specific space within the nucleus.

Question 24

How do chromosomes form a macromolecular structure?

Answer 24

This is achieved through the association of the DNA with DNA binding proteins such as histones to form complexes known as nucleosomes.

Question 25

What is the basic composition of a histone?

Answer 25

Made up of 8 histone subunits (pairs of H2A, H2B, H3 and H4) around which approx. 160 nucleotides can be wound around the protein twice.

Question 26

How is the 3D structure of chromatin formed?

Answer 26

Through the association of ring shaped proteins such as cohesin and the arrangement of nucleosomes

Question 27

What is the primary structure of the genome?

Answer 27

The double helix (incl DNA sequence, DNA methylation and genes)

Question 28

What is the secondary structure of the genome?

Answer 28

Nucleosomes (incl positioning, epigenetic modifications and chromatin accessibility)

Question 29

What makes up the 3D architecture of the genome?

in increasing magnitude

Answer 29

Chromatin loops (enabled by cohesin proteins, allow for promoter enhancer reactions)
Topologically associated domains (TADs, building blocks of functional units)
A/B compartments (A = active/euchromatin, B = inactive/heterochromatin, large scale chromatin state)
Chromosome territory (nuclear space occupied)
Nucleus (relative space between chromosome territories)

Question 30

What is an outdated model of chromosome and nucleosome structure?

Answer 30

The ‘beads on a string’ model - stacked nucleosomes into a 30nm fibre is merely an in vitro appearance of chromatin.