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

entire genetic material of an organism

Allele

different forms of the same gene

Structure of DNA

• double helix structure, two chains of nucleotides

• each nucleotide has a phosphate group and deoxyribose sugar (backbone)

• and a base

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

Male chromosomes