Analysing Evidence Topic 1

What does DNA stand for?

Deoxyribonucleic acid

What is DNA, in one sentence?

The genetic material of an organism

What shape does DNA form?

A double helix (two strands twisted together)

How many strands make up DNA, and how are they arranged?

Two strands, twisted together in a double helix

What is a single DNA strand called?

A polynucleotide (a polymer made of repeating nucleotide monomers)

What are the 3 parts of a nucleotide?

A deoxyribose sugar, a phosphate group, and a nitrogenous base

What makes up the DNA "backbone"?

The sugar (deoxyribose) and phosphate group — NOT the base

What are the 4 nitrogenous bases in DNA?

Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

What is complementary base pairing in DNA?

A pairs with T; G pairs with C

What type of bond holds the two DNA strands together?

A hydrogen bond

What does Chargaff's Rule state?

In DNA, A = T and G = C (across both strands of the molecule)

If Adenine makes up 40% of a DNA molecule's bases, what % is Thymine, and why?

40% — because A always equals T (Chargaff's Rule)

What is a gene?

A segment of DNA that codes for a protein/trait

What do genes ultimately code for?

Proteins, which control the structure and function of an organism

What is a genome?

The complete set of genes in an organism

What is a chromosome?

Compacted DNA

How many DNA molecules make up one chromosome?

1

Where are chromosomes found?

In the nucleus of eukaryotic cells

What are "linked genes"?

Genes located on the same chromosome

What is gene expression, in one sentence?

The process of using DNA to produce proteins

What are the two steps of gene expression, in order?

Transcription (DNA → mRNA), then Translation (mRNA → protein)

What is RNA, in one sentence?

A group of single-stranded nucleic acids involved in protein synthesis

What are the 3 parts of an RNA nucleotide?

Ribose sugar, phosphate group, nitrogenous base

Which base does RNA have that DNA doesn't?

Uracil (U), replacing Thymine

In RNA base pairing, what pairs with what?

A binds with U; G binds with C

Is RNA single-stranded or double-stranded?

Single-stranded

What sugar does RNA use, compared to DNA?

Ribose (DNA uses deoxyribose)

What is the objective of transcription?

To produce a strand of mRNA using a gene on DNA

Where does transcription take place?

The nucleus

What enzyme carries out transcription?

RNA polymerase

What does RNA polymerase do during transcription?

Binds to the template strand, unwinds the DNA by breaking hydrogen bonds, then builds mRNA using complementary base pairing

What is mRNA, and what does it do?

Messenger RNA — it exits the nucleus and acts as a template for building a protein

What is the objective of translation?

To produce a protein from mRNA

Where does translation take place?

The ribosome

What are proteins made of?

Chains of amino acids

What does tRNA do?

Transfer RNA carries amino acids to the mRNA, matching by complementary base pairing

What is a codon?

A sequence of 3 mRNA bases, which codes for 1 amino acid

What is the start codon, and which amino acid does it code for?

AUG, which codes for methionine

What are the 3 stop codons?

UAA, UAG, UGA

What is "redundancy" in the genetic code?

Multiple different codons can code for the same amino acid

Why must DNA replicate before a cell divides?

So each new cell gets a full, identical copy of the DNA

What is the correct order of DNA replication?

DNA unwinds → DNA helicase unzips the strands (breaks hydrogen bonds) → DNA polymerase adds complementary nucleotides to each template strand → 2 identical DNA molecules are formed

What enzyme unzips/unwinds DNA during replication?

DNA helicase

What enzyme adds new complementary nucleotides during replication?

DNA polymerase

Why is DNA replication called "semi-conservative"?

Each new DNA molecule keeps one original (old) strand and gains one newly synthesised strand

What is a mutation?

A heritable change in the structure or amount of genetic material (DNA)

What are the 3 types of point mutation?

Substitution, insertion, deletion

What is a substitution mutation?

One base is swapped for another

What is a silent mutation?

A mutation that does not change the resulting amino acid sequence

What is a missense mutation?

A mutation that results in a different amino acid, potentially changing protein function

What is a nonsense mutation?

A mutation that creates a premature stop codon, producing an unfinished, non-functioning protein

What is a frameshift mutation?

An insertion or deletion that shifts the entire reading frame downstream

What is a mutagen? Give 3 examples.

An environmental factor that increases mutation rate — e.g. chemicals, radiation, UV light

What type of mutation causes sickle cell disease?

A single base substitution, which changes haemoglobin's amino acid sequence

What causes Down Syndrome genetically?

A whole-chromosome mutation — 3 copies of chromosome 21 (trisomy 21), from a meiosis division error

What is mitosis?

Division of 1 diploid parent cell into 2 diploid daughter cells, identical to the parent

What is "diploid"?

Having 2 sets of chromosomes (pairs) — 46 in humans

What cells does mitosis occur in?

Somatic (body) cells

What are the two main purposes of mitosis?

Growth and development; replacing old or damaged cells

Must DNA replication happen before mitosis or meiosis?

Yes — before both, during interphase

What is meiosis?

Division of 1 diploid parent cell into 4 non-identical haploid daughter cells

What is "haploid"?

Having 1 set of chromosomes — 23 in humans

What cells does meiosis occur in, and why?

Germ-line cells, to form gametes (sex cells)

Why do gametes need to be haploid?

So that when two gametes fuse at fertilisation, each contributes 23 chromosomes to restore a diploid (46-chromosome) zygote

What happens in Meiosis I?

Separation of homologous chromosomes

What happens in Meiosis II?

Separation of sister chromatids

What is independent assortment?

Homologous chromosome pairs line up randomly on the equator during Meiosis I, creating different chromosome combinations

What is crossing over?

Exchange of DNA between homologous non-sister chromatids, increasing genetic variation

Name one similarity and one difference between mitosis and meiosis.

Similarity: DNA replicates before both. Difference: mitosis makes 2 identical diploid cells (growth/repair); meiosis makes 4 non-identical haploid cells (gametes)

What is an allele?

A different form/version of the same gene

How are dominant and recessive alleles written?

Dominant = uppercase letter; Recessive = lowercase letter

What is genotype?

An individual's allele combination for a trait (e.g. DD, Dd, dd)

What is phenotype?

The observable trait that results — depends on genotype AND environment

What happens when a dominant allele is present with a recessive allele?

The dominant allele masks the recessive allele

What is a homozygous genotype?

Two of the same allele (e.g. DD or dd)

What is a heterozygous genotype?

One of each allele (e.g. Dd) — sometimes called a "carrier"

What did Gregor Mendel's pea plant experiments establish?

The Law of Dominance, Law of Segregation, and Law of Independent Assortment

In a cross of Dd x dd, what is the genotype ratio of the offspring?

½ Dd : ½ dd

In a cross of Dd x dd, what is the phenotype ratio of the offspring?

½ detached earlobes : ½ attached earlobes

Are Punnett square outcomes guaranteed for any one offspring?

No — they show probabilities, not guarantees

Where are sex-linked (X-linked) traits coded?

By a gene on the X chromosome

How many X chromosomes does a female inherit? A male?

Female: 2 (XX); Male: 1 (XY, plus a Y)

Name three real conditions/traits coded on the sex chromosomes.

Colour blindness, haemophilia, (a form of) high blood pressure

Who is more likely to be affected by an X-linked DOMINANT trait, and why?

Females — because they have two X chromosomes, giving two chances to inherit the dominant allele

What fraction of possible female genotypes are affected by an X-linked dominant trait?

⅔ (XAXA and XAXa are affected; XaXa is not)

What fraction of possible male genotypes are affected by an X-linked dominant trait?

½ (XAY is affected; XaY is not)

Name a real example of an X-linked dominant condition.

Incontinentia pigmenti

Who is more likely to be affected by an X-linked RECESSIVE trait, and why?

Males — they only have 1 X chromosome, so there's no second allele to mask a recessive one

What fraction of possible female genotypes are affected by an X-linked recessive trait?

⅓ (only XaXa is affected; XAXA and XAXa are not)

What fraction of possible male genotypes are affected by an X-linked recessive trait?

½ (XaY is affected; XAY is not)

A colour-blind woman (XbXb) has children with a normal-vision man (XBY). Can they have a son with normal vision?

No — every son gets his only X chromosome from his mother, so all sons will be XbY (colour blind)

In the cross XBY x XbXb, what are all the daughters' genotype and phenotype?

XBXb — carriers with normal vision

What is the key rule that makes sex-linked inheritance different from autosomal inheritance?

The gene isn't always present as a matching pair — males have only 1 copy of X-linked genes, so outcomes differ by sex