2.4 Genome, Chromosomes, DNA and Genetics 🧬

Chromosomes

long strands of DNA found in the nucleus in functional pairs (except gametes/bacterial)

Gametes

only one chromosome from each pair (23)

Bacterial cells

only one chromosome in cytoplasm

Human number of chromosomes (usually)

46, in 23 pairs

Gene

short length of DNA found on a chromosome that codes for a particular characteristic or protein

Genome

all the genetic material of an organism

Allele

different forms of the same gene that can be dominant or recessive

Structure of DNA

• double helix structure, two chains of nucleotides

• each nucleotide has a phosphate and deoxyribose sugar backbone

• and complementary base pairing

How is DNA held together

phosphate and sugar molecules link together to form backbone, interlinking bases hold two strands together

Complementary base-pair rue

• Adenine (A) with thymine (T)

• Cytosine (C) with guanine (G)

DNA Code

each person has unique DNA/ sequence of bases which carries genetic code to make proteins

Base triplet hypothesis

three bases code for one amino acid which join to synthesise the protein

Mitosis

one division, produces 2 daughter cells that are genetically identical to parent/ each other (clones)

Uses for mitosis

growth, replace worn out cells, repair damaged tissue

Process for mitosis

• Each chromosome duplicates, splitting lengthwise into two identical chromosomes (joined by a centromere)

• Chromosomes line up along equator

• Chromosome and duplicate split towards opposite ends of cell

• Cell divides in two

• Two new daughter cells produced, same chromosomes as parent

Meiosis (reduction division)

two divisions, produces 4 daughter cells that are genetically different gametes (unique)

Where does meiosis take place

ovaries and testes

Process for meiosis

• Each chromosome duplicates, splitting lengthwise into two identical chromosomes (joined by a centromere)

• Chromosomes line up along equator

• Chromosome pairs move to opposite ends of cell

• Cell divides in two

• Two new daughter cells produced, half number chromosomes

• Chromosomes line up along cells equator

• Chromosome and their duplicate split towards opposite ends of cell

• Cell divides in two

• Four new daughter cells produced, genetically different

Independent assortment (+ random nature of fertilisation)

reassorts the chromosomes so there are millions of possible chromosome arrangements which causes variation

Dominant allele

always expressed when present, uppercase letter

Recessive allele

only expressed in absence of dominant, lowercase letter

Genotype

combination of alleles an organism has (Tt)

Phenotype

physical expression of a gene

Homozygous

both alleles are the same (TT or tt)

Heterozygous

two alleles are different (Tt)

Punnet square

grid used to determine frequency of genotypes in offspring

Monohybrid cross

shows characteristic controlled by a single gene with two alleles

Why are large numbers of offspring needed

to provide accurate ratios as fertilisation is random

Gregor Mendel concluded

• Characteristics are determined by factors within organism (genes)

• Factors can exist in two different forms (alleles)

• Factors separate during gamete formation (meiosis)

Test (back) cross

used to find the genotype by crossing with homozygous recessive and observing offspring

Interpreting test (back) cross

If no short plants are produced unknown parent was homozygous dominant, other wise heterozygous

Pedigree diagrams

used to show inheritance of genetic condition in family

Uses of pedigree diagrams

let people know if they are carriers of a condition

Genetic conditions

mutations, random changes in DNA or number of chromosomes causing fault in gene

Inheritance

genes are passed down from parents to offspring, this includes genetic conditions

Male chromosomes

XY

Female chromosomes

XX

Eggs

all contain one x chromosome

Sperm

half contain either one x or one y

Sex-linkage

always inherit an x chromosome from mother + an x or y from father

Haemophilia

blood disorder where people can’t clot their blood, leads to excessive bleeding

Haemophilia sex-linkage

caused by recessive allele on x chromosome

Why is haemophilia more common in males

they only need one recessive allele as they only have one x chromosome

Carrier

homozygous, do not experience symptoms but can pass on their recessive allele

Cystic fibrosis

mainly affects lungs and digestive system, clogged with mucus or frequent infections

Inheritance of cystic fibrosis

recessive allele

Huntington’s disease

affects nerve cells in brain leading to brain damage, fatal

Karyotype

shows full set of chromosomes in individual

Inheritance of Huntington’s disease

dominant allele, doesn’t usually isn’t shown until middle aged by which they may have already passed it on

Down’s syndrome

caused reduced muscle tone and learning difficulties

Cause of down syndrome

extra chromosome at number 21 (trisomy)

Inheritance of down’s syndrome

happens by chance during formation of egg cells (24 chromosomes)

Genetic screening

testing people or groups for the presence of a particular allele/ abnormality

Testing for down syndrome

• blood test at 10-14 weeks to assess risk

• some at risk are offered amniocentesis

Risk of amniocentesis

1% risk of having a miscarriage

Amniocentesis procedure

• needle inserted into amniotic fluid surrounding foetus

• fluid is withdrawn

• foetal cells are examined for genetic conditions

People commonly offered amniocentesis

• history of genetic conditions

• previous birth with genetic condition

• older mothers

• problems identified in earlier tests e.g blood test

Ethics of genetic screening

• People should have the choice to be tested

• Decisions for unborn babies are made by parents

Amniocentesis vs blood tests

• Amniocentesis is more accurate but involves a small risk of miscarriage

• Blood tests are safer but less accurate, often needing further testing

Abortion after genetic screening

Advantages	Disadvantages
prevents suffering, poor quality of life and eases burden on loved ones	unborn child has a right to life and cannot voice their choice
time spent caring for child with abnormality	abortion is against some religious teachings/ illegal in some countries

Reasons to get genetic screening

could show you are a carrier, children could have a chance of inheriting condition and this may persuade people to adopt

Genetic information

• could advance medical research

• however insurers/ employers might misuse it, causing discrimination

• strong laws are needed to protect privacy

Genetic engineering

deliberately changing genome of an organism to introduce desirable traits

Uses of genetic engineering

medicine, agriculture and scientific research

Production of human insulin

human inulin gene is inserted into genome of bacteria and grown

Process of producing human insulin

1. human insulin gene is removed using restriction enzyme

2. bacterial plasmid is cut open using same enzyme

3. restriction enzymes cut DNA creating complementary ‘sticky ends’ that join by base pairing

4. different enzyme is used to join insulin gene and bacterial plasmid

5. plasmid is placed back into bacterial cell, now genetically modified bacterium

6. cells grown in fermenter, rapidly reproduce

7. produce large quantities of insulin under optimal conditions

8. downstreaming occurs

Optimal conditions for insulin production

warmth, moisture and oxygen

Downstreaming

insulin is extracted purified and packaged to ensure it is safe and effective for treating diabetes

Advantages of genetically engineered insulin

• no side effects or allergies

• no animals killed/ ethical issues

• large quantities can be made quickly and cheaply

• no rejection as its human insulin