Genetics: Topic 3.1 and 3.2 Genes and Chromosomes

Question 1

Define Gene

Answer 1

A gene is a section or length of DNA that codes for a polypeptide
Genes are heritable factors that influence specific characteristics (via the polypeptides produced)

Question 2

What is a gene locus?

Answer 2

The location of a gene on a chromosome is known as its locus

Question 3

State the specificity of a locus

Answer 3

Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome

Question 4

What is the number of genes in an organism not related to?

Answer 4

The number of genes a species has is not related to the size/complexity or even the sophistication of the organism
Because genes can vary in length

Question 5

Define alleles

Answer 5

Alternative forms of a gene can exist, these various specific forms are called alleles

Question 6

Outline two characteristics concerning the concept of alleles

Answer 6

Although alleles are different forms of the same gene, they all still occupy the same locus on the chromosome
New alleles occur through mutations

Question 7

How alleles lead to variation?

Answer 7

Multiple alleles can exist for a gene that determines a specific trait
Each allele results in a different variation of that trait
Eg. blood types A, B, AB and O

Question 8

Explain the number alleles for a gene in eukaryotic chromosomes

Answer 8

The chromosomes of eukaryotic cells occur in homologous pairs (there are two copies of each chromosome, one copy inherited from each parent) which means that cells have two copies of every gene
As a result, a cell possesses two alleles of every gene within its nucleus

Question 9

Explain homozygous and heterozygous alleles

Answer 9

When the two alleles at a locus are the same/identical they are described as homozygous
When the two alleles at a locus are different they are described as heterozygous

Question 10

How do alleles differ from each other?

Answer 10

Alleles differ from each other by one or only a few bases
Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype

Question 11

What still remains common between alleles of a gene?

Answer 11

Even though different alleles of a gene have slightly different base sequences, they still occupy the same locus on the chromosome

Question 12

What is SNP when talking about alleles?

Answer 12

The exact positions where bases differ between alleles are called SNPs or snips (Single Nucleotide Polymorphisms)
An allele can have several snips but still only differ by a few bases from its other allele

Question 13

Define gene mutation

Answer 13

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait. New alleles are formed by mutation

Question 14

State the nature of gene mutation

Answer 14

Mutations occur all the time and at random

Question 15

Discuss the types of effects gene mutations can have

Answer 15

Gene mutations can be beneficial, detrimental or neutral

Beneficial mutations change the gene sequence to create new variations of a trait
Detrimental mutations truncate the gene sequence (shorten it by producing a STOP codon) to abrogate the normal function of a trait
Neutral mutations have no effect on the functioning of the specific feature (silent mutations)

Question 16

Discuss inheritance of mutation

Answer 16

Mutations present in normal body cells are not inherited, they are eliminated from the population once those cells die

Mutations within gametes are inherited by offspring, possibly causing genetic disease

Question 17

What is substitution mutations

Answer 17

A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base is known as a substitution mutation

Question 18

Explain the result of substitution mutation

Answer 18

A substitution mutation will only change the amino acid for the triplet (group of three consecutive bases) where the mutation occurs; it will not have a knock-on effect further along the gene/polypeptide

Question 19

What is the cause of sickle cell anaemia?

Answer 19

Sickle cell anaemia is a genetic disease caused by a single base substitution mutation within the gene (Hb) that codes for the alpha-globin polypeptide in haemoglobin

Question 20

Explain the mutation that occurs in sickle cell anaemia

Answer 20

Within the haemoglobin gene, the base thymine (T) is replaced by the base adenine (A). This causes the DNA triplet GAG to mutate to GTG
The mutated DNA codon GTG is transcribed into the mRNA codon GUG, instead of GAG
During translation the amino acid valine (VAL) replaces the original amino acid glutamic acid (GLU); this occurs on the sixth position of the polypeptide
The slightly different polypeptide results in a new allele, HbS

Question 21

Explain the effects of sickle cell anaemia

Answer 21

The protein haemoglobin S is produced instead of haemoglobin A; this causes a distortion in the shape of the red blood cells into sickle shapes
Sickle-shaped red blood cells:
Have a limited oxygen-carrying capacity
Block the capillaries limiting the flow of normal red blood cells
People with sickle cell anaemia suffer from acute pain, fatigue and anaemia

Question 22

Explain what is a genome and what all does it include in eukaryotes and prokaryotes

Answer 22

The total of all the genetic information in an organism is called the genome of the organism
This is a complete set of genes present within every cell of an organism
This includes all genes as well as non-coding DNA sequences
Mitochondrial DNA and chloroplast DNA are included in the genome
In a prokaryote cell, plasmid DNA is included in its genome

Question 23

Explain chromosome arrangement of feature in Prokaryotes

Answer 23

Prokaryotes do not possess a nucleus – instead genetic material is found free in the cytoplasm in a region called the nucleoid

The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)

The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging

Question 24

What are plasmids?

Answer 24

prokaryotic cells may possess additional circular DNA molecules called plasmids that are independent
They usually only contain a few genes
They are short, typically 100,000 base pairs in length

Question 25

Advantages or uses of plasmids

Answer 25

They are more accessible for proteins required for gene expression and therefore contain genes that are required often, quickly and/or in emergencies
Plasmids can sometimes be passed ‘sideways’ from one cell to another via sex pilli outside of the normal inheritance pattern during cell division (bacterial conjugation)
As plasmids can self-replicate and autonomously synthesise proteins, they are ideal vectors during genetic engineering to transfer DNA between species

Question 26

Explain the general eukaryotic chromosomes

Answer 26

The genetic material of eukaryotic cells consist of multiple linear molecules of DNA that are associated with histone proteins

Question 27

What are histone proteins and its purpose

Answer 27

The main proteins present are the large, positively charged globular proteins called histones, their role is to organise and wrap the DNA tightly so that it fits into the nucleus

Question 28

Explain organisation of chromosomes in eukaryotic cells

Answer 28

DNA is complexed with eight histone proteins to form a complex called a nucleosome.The DNA is attracted to the histones because DNA is negatively charged and the histones are positively charged.

Between the nucleosomes is a single
string of DNA. There is often a fifth type of histone attached to the linking string of
DNA near each nucleosome. This fifth histone leads to further wrapping (packaging).

These then coil to form a solenoid structure which is condensed to form a 30 nm fibre.
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin.

Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope

Question 29

How do chromosomes differ from each other?

Answer 29

Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere
Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus

Question 30

Outline features of homologous chromosomes

Answer 30

The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)

Question 31

What helps in alignment of homologous chromosomes in metaphase during meiosis

Answer 31

Having the same genes in the same loci helps homologous chromosomes line up alongside each other during Metaphase 1 of meiosis

Question 32

What is the advantage of a cell being diploid?

Answer 32

Having two alleles gives some protection from harmful mutations that are recessive as there is a copy of the correctly-functioning allele still present

Question 33

How is sex determined in mammal species

Answer 33

Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)
Females have the sex chromosomes (pair 23 in humans) XX
Males have the sex chromosomes (pair 23 in humans) XY

Question 34

What are autosomes

Answer 34

All other chromosomes (pairs 1 - 22 in humans) are autosomes and have no influence on determining the sex of offspring

Question 35

Which parent is responsible for determining sex of the child and how

Answer 35

Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
Due to meiosis, half of his sperm cells will carry his X chromosome, half his Y chromosome
The chromosome carried by the sperm that fertilizes the egg will determine the sex of the child
His daughters receive a copy of his X chromosome
His sons receive a copy of his Y chromosome

Question 36

Outline differences between X and Y chromosome in terms of number of genes

Answer 36

The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome
Fewer genes are coded for on the Y chromosome as a result
The X carries around 16 × more genes than the Y chromosome

Question 37

What genes X chromosome codes for

Answer 37

Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y

Question 38

What is the function of Y chromosome

Answer 38

The Y chromosome carries genes that code for male characteristics
One of these genes is the SRY gene which is involved in
Development of testes in male embryos
Production of testosterone
In its absence of a Y chromosome, female sex organs will develop
The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

Question 39

What is a karyogram

Answer 39

A karyogram will show all the chromosomes in homologous pairs, starting with the longest pair and ending with the shortest (with sex chromosomes shown last)

Question 40

How are karyograms obtained

Answer 40

Harvesting cells (usually from a foetus or white blood cells of adults)
Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed This is most notable in metaphase
The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not

The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram

Question 41

Karyotyping will typically occur prenatally and is used to:

Answer 41

Determine the gender of the unborn child (via identification of the sex chromosomes)
Test for chromosomal abnormalities

Question 42

What are Chromosome mutations

Answer 42

Chromosome mutations involve a change in the number of chromosomes
A spontaneous chromosome mutation called non-disjunction occurs when chromosomes fail to separate during meiosis

Question 43

Explain consequences of chromosome mutation

Answer 43

The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
Many such gametes will form a non-viable embryo that aborts before becoming a foetus, but not always
If the abnormal gametes combine in viable fertilization, then a chromosome mutation occurs as the diploid cell will have the incorrect number of chromosomes

Question 44

Explain Down syndrome as an example of chromosome mutation

Answer 44

An example of chromosome mutation is Down syndrome
Individuals with Down syndrome have a total of 47 chromosomes in their genome as they have three copies of chromosome 21
Down syndrome is also called Trisomy 21
Symptoms include distinctive facial features, hearing loss, learning and growth impairment