Unit 4 Topic 1: DNA, genes, and the continuity of life

explain DNA

deoxyribonucleic acid is a double-stranded molecule that occurs bound to proteins in chromosomes in the nucleus, and as unbound circular DNA in the cytosol of prokaryotes, and in the mitochondria and chloroplasts of eukaryotic cells

Explain the role of helicase and DNA polymerase in the process of DNA replication; refer to the direction of replication.

Helicase unwinds the DNA by breaking hydrogen bonds, forming a replication fork, and DNA polymerase builds new strands by adding complementary nucleotides in the 5' to 3' direction.

recall the structure of DNA of Nucleotide Composition

DNA is made of nucleotides, each with:

1. A phosphate group

2. A sugar (deoxyribose)

3. A nitrogen base – A, T, G, or C

recall the structure of DNA of Complementary Base Pairing

DNA has two strands that pair specifically:

• Adenine (A) → Thymine (T) (2 hydrogen bonds)

• Guanine (G) → Cytosine (C) (3 hydrogen bonds)

recall the structure of DNA of Hydrogen Bonds

• The strands are held together by weak hydrogen bonds between bases.

• These bonds are strong enough to hold DNA together but can break for replication or transcription.

within the process of meiosis I and II recognise the role of homologous chromosomes

exchange genetic material, and then segregate to ensure each gamete receives one complete set of chromosomes, promoting genetic diversity.

Explain the process of crossing over and recombination. How does crossing over contribute to genetic variation?

Crossing over is a process during meiosis where homologous chromosomes exchange genetic material, resulting in recombination and increased genetic variation. This exchange occurs between non-sister chromatids of paired homologous chromosomes during prophase I, creating new combinations of alleles on the chromosomes.

describe the process of spermatogenesis (diploid cells).

Occurs in testes

Starts at puberty, keeps going

Diploid cells divide and make 4 sperm cells

Sperm are small and can move

describe the process of oogenesis (haploid cells).

Occurs in ovaries

Starts before birth, pauses, then resumes at puberty, stops at menopause

Diploid cells divide and make 1 egg and 3 polar bodies

Eggs are large, don’t move, and store nutrients

define genome

complete set of DNA instructions within a cell

define gene

the basic unit of heredity

define exons

the coding regions of a gene, meaning they are transcribed into mRNA and translated into proteins.

define introns

are non-coding intervening sequences that are removed from the RNA transcript before translation.

explain the process of protein synthesis in terms of transcription of a gene into messenger RNA in the nucleus

During transcription, RNA polymerase copies the DNA template of a gene into a complementary messenger RNA (mRNA) strand in the nucleus, which then carries the genetic code to the cytoplasm for protein synthesis.

explain the process of protein synthesis in terms of translation of mRNA into an amino acid sequence at the ribosome (refer to transfer RNA, codons and anticodons)

During translation, the ribosome reads the mRNA codons, and transfer RNA (tRNA) molecules with complementary anticodons deliver specific amino acids, which are joined together in sequence to form a protein.

what is the process of gene expression

is to synthesise a functional gene product (protein or functional RNA); that the process can be regulated and is used by all known life

What factors regulate the phenotypic expression of genes?

regulated by proteins that bind to specific DNA sequences during transcription and translation, by the products of other genes, and by environmental influences such as those studied through twin epigenetic research.

How does differential gene expression, controlled by transcription factors, regulate cell differentiation for tissue formation and morphology?

Differential gene expression, regulated by transcription factors, activates or silences specific genes in cells, allowing them to develop specialised functions, which leads to tissue formation and overall body morphology.

recall an example of a transcription factor gene that regulates morphology

The HOX transcription factor family regulates morphology by controlling the body plan and positioning of structures during embryonic development.

recall an example of a transcription factor gene that regulates cell differentiation

The sex-determining region Y (SRY) gene regulates cell differentiation by triggering the development of male reproductive organs.

identify how mutations in genes and chromosomes can result from errors in DNA replication (point and frameshift mutation)

• Point mutation → one base is changed → may alter one amino acid.

• Frameshift mutation → bases are added or deleted → shifts the reading frame → changes many amino acids.

identify how mutations in genes and chromosomes can result from errors in cell division (non-disjunction)

• Chromosomes don’t separate properly during meiosis/mitosis.

• Leads to cells with the wrong number of chromosomes.

• Example: Down syndrome (extra chromosome 21).

identify how mutations in genes and chromosomes can result from errors in damage by mutagens (physical, including UV radiation, ionising radiation and heat and chemical)

Mutagens are things that damage DNA:

• Physical:

  • UV radiation → causes abnormal bonding in DNA.

  • Ionising radiation → breaks DNA strands.

  • Heat → destabilises DNA.

• Chemical:

  • From smoke, pesticides, etc. → change DNA bases or cause insertions/deletions.

explain how non-disjunction leads to aneuploidy

Non-disjunction occurs when chromosomes fail to separate properly during cell division, resulting in gametes with too many or too few chromosomes, which causes aneuploidy.

describe how inherited mutations can alter the variations in the genotype of offspring.

Inherited mutations change the DNA sequence passed from parents to offspring, introducing new genetic variations that can alter the offspring’s genotype.

define polygenic inheritance and predict frequencies of genotypes and phenotypes for using three of the possible alleles.

Polygenic inheritance occurs when a trait is controlled by multiple genes, and with three possible alleles, combining them creates a range of genotypes that produce a continuous variation of phenotypes, often forming a bell-shaped distribution.

describe the process of making recombinant DNA

o    isolation of DNA, cutting of DNA (restriction enzymes)

o    insertion of DNA fragment (plasmid vector)

o    joining of DNA (DNA ligase)

o    amplification of recombinant DNA (bacterial transformation)

explain DNA sequencing

Determines the exact order of nucleotides in an organism’s DNA, which helps map species’ genomes — used to study genes, understand evolution, and develop medical treatments.

explain DNA profiling

Analyses specific regions of DNA to identify unique genetic information — used in forensics, paternity testing, and identifying individuals or species.

explain the purpose of polymerase chain reaction (PCR) and gel electrophoresis

amplify specific DNA segments, while gel electrophoresis separates and analyses DNA fragments based on their size.