DNA, RNA, protein synthesis, meiosis

Structure and function of DNA - 7 points

• Double helix shape with the sugar-phosphate backbone on the outside - protects the nitrogenous bases on the inside

• The two strands are complementary and are joined together by weak hydrogen bonds - the two strands can separate easily to synthesise mRNA in transcription and for (semi-conservative) DNA replication

• Many weak hydrogen bonds between bases (and higher proportion of G-C pairings) - the double helix is stable

• Very stable structure - mutations rarely happen and genetic information can be passed down from generation to generation

• Extremely large molecule - carries an immense amount of genetic information

• Specific sequence of base pairs - codes for many specific proteins

• Compact and helical - can be stored in the nucleus - a small space

Structure of DNA

DNA is a double-stranded polymer, coiled into a double helix. Each strand is made up of monomers called nucleotides, joined together by phosphodiester bonds. Each nucleotide comprises a phosphate group, bonded to a pentose sugar (deoxyribose), bonded to a nitrogenous base. The two strands are joined together by hydrogen bonds between complementary bases - adenine joins to thymine with two hydrogen bonds and cytosine to guanine with three.

Reaction that joins nucleotides

Condensation

Bond between nucleotides

Phosphodiester

Bond between phosphate group and pentose sugar

Ester

Bond between a pentose sugar and nitrogenous base

Glycosidic

Why is DNA anti-parallel

The two strands in DNA run in opposite directions - on one strand, the 5 prime end of the deoxyribose sugar is above the 3 prime end, and on the other strand the 3 prime end is above the 5 prime end

Purpose of RNA

mRNA - transfers genetic information from DNA to the ribosomes for protein synthesis. tRNA - transfers amino acids to the ribosomes to be joined together in the correct order (determined by the codons on the mRNA) into a polypeptide by the formation of peptide bonds

Structure of tRNA

• Single strand of RNA

• Folded into a clover-leaf shape

• Hydrogen bonds between complementary base pairs

• Binding site for an amino acid

• Anticodon to bind with hydrogen bonds to a codon on mRNA

Purpose of semi-conservative replication of DNA

Ensures genetic continuity between generations of cells because it ensures that daughter cells produced in mitosis inherit all the genes from their parent cell

Process of DNA replication

• Semi-conservative replication

• DNA helicase moves along the molecule of DNA, causing the molecule to unwind and breaking the hydrogen bonds between the complementary bases on the two strands so that the strands separate

• Each strand acts as a template: nucleotides with bases complementary to the exposed bases join to the exposed bases on each strand with hydrogen bonds (A to T with two and C to G with three)

• DNA polymerase moves along the new strand of DNA and joins the nucleotides together by catalysing the formation of phosphodiester bonds between them

Proteome

The full range of proteins produced by the genome

Splicing

The pre-mRNA molecule is folded so that the introns form loops. The introns are then spliced out of the pre-mRNA by a spliceosome, and the exons are joined back together to form the final mRNA molecule.

Where does the ribosome attach to the mRNA

start codon

how does a peptide bond form during translation

using an enzyme and energy released from the hydrolysis of ATP

chromosome mutations

• non-disjunction when individual homologous pairs do not separate during meiosis, leading to a gamete having one more or one fewer of one chromosome, e.g. trisomy 21 which causes Down syndrome

• polyploidy - where individuals have three or more sets of chromosomes, usually occurring in plants, occurring when chromosomes do not form two distinct sets in meiosis, forming diploid gametes, which then fuse - either one haploid and one diploid (forming a triploid) or two diploids (forming a tetraploid)

Prophase 1

• DNA condenses and becomes visible as chromosomes (each chromosome consists of two sister chromatids joined together by a centromere)

• Chromosomes arranged side by side in homologous pairs - pair of chromosomes: bivalent

• Crossing over of alleles between non-sister chromatids in the chromosomes of the homologous pairs - crossing over point is chiasma

• Centrioles migrate to opposite poles and the spindle is formed

• Nuclear envelope breaks down, nucleolus disintegrates

Metaphase 1

bivalents (pairs of homologous chromosomes) line up along the equator of the spindle - there is independent assortment/segregation - spindle fibres attached to centromeres

Anaphase 1

homologous pairs of chromosomes are separated as spindle fibres contract and microtubules pull whole chromosomes to opposite ends of the spindle

telophase 1

chromosomes arrive at opposite poles, spindle fibres start to break down, nuclear envelopes and nucleoli reform, organelles are evenly distributed and cytokinesis occurs - in animals, cell membrane pinches inwards; in plants, vesicles from the golgi apparatus line up along the equator and fuse, and the middle lamella is formed, which then has layers of cellulose laid upon it to form the cell wall. Two haploid cells are produced.

Prophase 2

Nuclear envelope breaks down, chromosomes condense, new spindle forms at a right angle to the old one

metaphase 2

Chromosomes line up in single file along the equator. Spindle fibres attach to chromosomes using the centromere

anaphase 2

centromeres divide and individual chromatids are pulled to opposite poles

telophase 2

nuclear membranes form around each group of chromosomes. Cytokinesis

process of transcription

• Hydrogen bonds between complementary DNA bases between the two strands break (due to DNA helicase)

• One DNA strand acts as a template

• Free RNA nucleotides align along the template strand by complementary base pairing

• In RNA, uracil is used instead of thymine to pair with adenine on DNA

• RNA polymerase joins together adjacent RNA nucleotides

• By forming phosphodiester bonds

• Pre-mRNA is spliced to form mRNA

process of translation

• mRNA attaches to ribosomes

• tRNA anticodons bind to complementary mRNA codons

• Each tRNA brings a specific amino acid

• Amino acids join by peptide bonds

• Formed using ATP (and an enzyme)

• tRNA released after amino acid joined to polypeptide

• Ribosome moves along the mRNA to form the polypeptide

Process of crossing over

• Homologous pairs of chromosomes associated (form a bivalent)

• Chiasmata form

• Equal lengths of non-sister chromatids / alleles are exchanged

• Producing new combinations of alleles