Information flow

Semiconservative replication

DNA replication where each daughter DNA molecule contains one original (parental) strand and one newly synthesised strand

Meselson-Stahl experiment

experiment using heavy (¹⁵N) and light (¹⁴N) nitrogen to prove DNA replication is semiconservative

Result of 1 replication (Meselson-Stahl)

hybrid DNA (half heavy

Result of 2 replications (Meselson-Stahl)

mixture of hybrid and light DNA

Basic DNA replication

DNA strands separate

Template strand

original DNA strand used to build a new complementary strand

DNA synthesis direction

DNA is always synthesised in the 5’ to 3’ direction

Leading strand

strand synthesised continuously in the 5’ to 3’ direction

Lagging strand

strand synthesised discontinuously in fragments due to 5’ to 3’ constraint

Primer

short RNA sequence that provides a starting point for DNA synthesis

Reason primers are needed

DNA polymerase cannot start synthesis and can only add to an existing 3’ end

Helicase

enzyme that breaks hydrogen bonds and separates DNA strands

Primase

enzyme that synthesises RNA primers

DNA polymerase

enzyme that builds new DNA strands and proofreads

Single-strand binding proteins

proteins that stabilise separated DNA strands and prevent re-annealing

Topoisomerase

enzyme that relieves tension and prevents supercoiling ahead of replication fork

Ligase

enzyme that joins Okazaki fragments together

Okazaki fragments

short DNA fragments formed on the lagging strand

Lagging strand replication

DNA made in fragments because synthesis is only 5’ to 3’

Unwinding step

DNA strands separated by helicase and stabilised by binding proteins

Primer addition

RNA primer added by primase

DNA synthesis step

DNA polymerase extends DNA from primer in 5’ to 3’ direction

Primer removal

RNA primers removed and replaced with DNA

Fragment joining

DNA ligase joins Okazaki fragments

Antiparallel DNA

two DNA strands run in opposite directions (5’→3’ and 3’→5’)

Central dogma

flow of genetic information from DNA to RNA to protein

DNA replication (central dogma)

DNA is copied into DNA

Transcription (central dogma)

DNA is transcribed into RNA

Translation (central dogma)

RNA is translated into protein

Transcription

process of making mRNA from DNA

Location of transcription

nucleus in eukaryotes

Template strand (transcription)

DNA strand read to make mRNA

RNA base pairing

A→U

Coding strand

DNA strand with same sequence as mRNA (except T replaced by U)

Template strand direction

read 3’ to 5’

mRNA synthesis direction

RNA is synthesised 5’ to 3’

RNA polymerase

enzyme that adds RNA nucleotides during transcription

RNA processing

modification of mRNA after transcription in eukaryotes

5’ cap

modification that protects mRNA

Poly-A tail

modification that increases mRNA stability

Introns

non-coding regions removed during RNA processing

Exons

coding regions joined together after splicing

Replication vs transcription

replication copies entire genome (DNA→DNA) while transcription copies a gene (DNA→RNA)

Translation vs transcription

translation makes protein from RNA while transcription makes RNA from DNA

Translation location

occurs at ribosome

tRNA function

carries amino acids and matches codons during translation

DNA template to mRNA step 1

identify template strand (3’→5’)

DNA template to mRNA step 2

write complementary RNA bases

DNA template to mRNA step 3

replace T with U

DNA template to mRNA step 4

ensure final strand is 5’→3’

G1 phase

cell grows

S phase

DNA is replicated and chromosomes duplicated into sister chromatids

G2 phase

cell grows further and prepares for mitosis

Sister chromatids

identical copies of a chromosome joined at the centromere

Interphase

G1 + S + G2 phases where cell grows

Chromatin

uncondensed form of chromosomes during interphase

Prophase

chromosomes condense

Metaphase

chromosomes align at the middle of the cell

Anaphase

sister chromatids separate and move to opposite poles

Telophase

nuclei reform and chromosomes decondense

Cytokinesis

division of cytoplasm to form two identical daughter cells

Cleavage furrow

indentation that forms during cytokinesis in animal cells

Order of mitosis stages

prophase → metaphase → anaphase → telophase → cytokinesis

What is a gene?

A segment of DNA that codes for a functional product (protein or RNA)

What is the product of a gene?

A protein or functional RNA molecule

What process(es) are used to make a gene product?

Transcription (DNA → RNA) and translation (RNA → protein)

What is an allele?

A different version of the same gene

How might the products of two different alleles differ?

They may produce proteins with different amino acid sequences leading to different functions or no function

Homozygous dominant genotype (B allele example)

BB produces blue flowers

Heterozygous genotype (B allele example)

Bb produces blue flowers

Homozygous recessive genotype (B allele example)

bb produces white flowers

Where does meiosis occur?

In the gonads

What does meiosis produce?

Gametes

Cells with 2n chromosomes are called what?

Diploid

Cells with n chromosomes are called what?

Haploid

In animals

what are haploid cells called?

What are chromosome pairs in diploid cells called?

Homologous chromosomes

What is the purpose of meiosis?

To reduce chromosome number by half and produce genetically diverse gametes

Does DNA replication occur before meiosis?

Yes

What happens in meiosis I?

Homologous chromosomes pair

What happens in meiosis II?

Sister chromatids separate

What happens in prophase I?

Homologous chromosomes pair and crossing over occurs

What happens in metaphase I?

Homologous pairs line up randomly at the equator

What happens in anaphase I?

Homologous chromosomes separate

What happens in telophase I?

Two haploid cells form

What happens in prophase II?

Chromosomes condense again

What happens in metaphase II?

Chromosomes line up at the equator

What happens in anaphase II?

Sister chromatids separate

What happens in telophase II?

Four haploid gametes form

When does crossing over occur?

Prophase I

What is crossing over?

Exchange of genetic material between homologous chromosomes

Do maternal and paternal chromosomes separate evenly by colour?

No

What is the consequence of this?

Genetic variation (independent assortment)

When do homologous chromosomes separate?

Anaphase I

What is the location of a gene on a chromosome called?

Locus

What is a gene made of?

DNA

How many alleles of a gene are on each chromosome?

One

How many alleles can a diploid individual have for one gene?

Two

How are new alleles formed?

Through mutation

Do gametes contain two copies of each gene?

No